Fix various warnings when building without libusb.

Fix various warnings when building without libusb.

  ../src/backend.c: In function 'sr_init':
  ../src/backend.c:435:1: warning: label 'done' defined but not used [-Wunused-label]
   done:
   ^

  ../src/device.c: In function 'sr_dev_inst_connid_get':
  ../src/device.c:525:7: warning: unused variable 'connection_id' [-Wunused-variable]
    char connection_id[64];
         ^
  ../src/device.c:524:20: warning: unused variable 'b' [-Wunused-variable]
    int r, cnt, i, a, b;
                      ^
  ../src/device.c:524:17: warning: unused variable 'a' [-Wunused-variable]
    int r, cnt, i, a, b;
                   ^
  ../src/device.c:524:14: warning: unused variable 'i' [-Wunused-variable]
    int r, cnt, i, a, b;
                ^
  ../src/device.c:524:9: warning: unused variable 'cnt' [-Wunused-variable]
    int r, cnt, i, a, b;
           ^
  ../src/device.c:524:6: warning: unused variable 'r' [-Wunused-variable]
    int r, cnt, i, a, b;
        ^
  ../src/device.c:523:22: warning: unused variable 'drvc' [-Wunused-variable]
    struct drv_context *drvc;
                      ^