libbladeRF: Addressed items identified by Clang scan

tests/test_sync/src/test.c:
    Changed sample buffer type to int16_t. These alleivates noise from static
    analysis tools regarding a type mismatch with sizeof(), and better
    exemplifies the buffers that should be used in conjunction with the SC16 Q11
    format.

tests/test_repeater/src/repeater.c:
    Fixed unchecked pthread_cond_wait result.

tests/test_async/main.c:
    Fixed unchecked return values, minor cleanup.

src/lms.c:
    Removed dead assignment.

src/backend/usb/usb.c:
    Corrected unchecked return value.

src/backuend/usb/libusb.c:
    Fixed uninitialized variable in error path.