saleae-logic-pro: Fix two compiler warnings.

saleae-logic-pro: Fix two compiler warnings.

  src/hardware/saleae-logic-pro/protocol.c:389:12: warning: 'set_led' defined but not used [-Wunused-function]
   static int set_led(const struct sr_dev_inst *sdi, uint8_t r, uint8_t g, uint8_t b)
              ^
    CC       src/hardware/saleae-logic-pro/api.lo
  src/hardware/saleae-logic-pro/api.c: In function 'dev_acquisition_handle':
  src/hardware/saleae-logic-pro/api.c:332:9: warning: missing initializer for field 'tv_sec' of 'struct timeval' [-Wmissing-field-initializers]
    struct timeval tv = {};
           ^
  In file included from /usr/include/x86_64-linux-gnu/sys/time.h:27:0,
                   from include/libsigrok/libsigrok.h:24,
                   from src/hardware/saleae-logic-pro/protocol.h:25,
                   from src/hardware/saleae-logic-pro/api.c:23:
  /usr/include/x86_64-linux-gnu/bits/time.h:32:14: note: 'tv_sec' declared here
       __time_t tv_sec;  /* Seconds.  */
                ^