libbladeRF: fix minor comment typos
libbladeRF: correct RXVGA2-related bladerf_get/set_sampling methods

bladerf_get_sampling - changed case value so BLADERF_SAMPLING_EXTERNAL is
    reported correctly.

bladerf_set_sampling - for internal/external sampling, the RXVGA2
    log warning messages were reversed (enable/disable RXVGA2)