Fix a few "variable set but not used" compiler warnings.

Fix a few "variable set but not used" compiler warnings.

../src/hardware/appa-55ii/api.c: In function ‘scan’:
../src/hardware/appa-55ii/api.c:47:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/arachnid-labs-re-load-pro/api.c: In function ‘scan’:
../src/hardware/arachnid-labs-re-load-pro/api.c:62:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/atten-pps3xxx/api.c: In function ‘scan’:
../src/hardware/atten-pps3xxx/api.c:81:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/brymen-dmm/api.c: In function ‘scan’:
../src/hardware/brymen-dmm/api.c:89:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/cem-dt-885x/api.c: In function ‘scan’:
../src/hardware/cem-dt-885x/api.c:74:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/conrad-digi-35-cpu/api.c: In function ‘scan’:
../src/hardware/conrad-digi-35-cpu/api.c:45:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/demo/demo.c: In function ‘scan’:
../src/hardware/demo/demo.c:255:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/fluke-dmm/api.c: In function ‘fluke_scan’:
../src/hardware/fluke-dmm/api.c:64:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/gmc-mh-1x-2x/api.c: In function ‘scan_1x_2x_rs232’:
../src/hardware/gmc-mh-1x-2x/api.c:147:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/gmc-mh-1x-2x/api.c: In function ‘scan_2x_bd232’:
../src/hardware/gmc-mh-1x-2x/api.c:234:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/ikalogic-scanaplus/api.c: In function ‘scan’:
../src/hardware/ikalogic-scanaplus/api.c:69:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^
../src/hardware/tondaj-sl-814/api.c: In function ‘scan’:
../src/hardware/tondaj-sl-814/api.c:44:22: warning: variable ‘drvc’ set but not used [-Wunused-but-set-variable]
  struct drv_context *drvc;
                      ^