hwdriver.c: Fix two scan-build warnings.

hwdriver.c: Fix two scan-build warnings.

  src/hwdriver.c: In function ‘log_key’:
  src/hwdriver.c:648:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    opstr = op == SR_CONF_GET ? "get" : op == SR_CONF_SET ? "set" : "list";
               ^~
  src/hwdriver.c: In function ‘check_key’:
  src/hwdriver.c:681:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    opstr = op == SR_CONF_GET ? "get" : op == SR_CONF_SET ? "set" : "list";
               ^~