libbladeRF: Moved internal/external sampling logic to lms.c

The "top level" API function implementations in bladerf.c are intended
to be fairly thin to allow for some future efforts:
    - Pulling files such as lms.c and si5338.c into NIOS code
    - Wrapping per-device locks around calls in bladeRF.c in some
      upcoming thread safety changes.

As such, the logic involved with switching between internal & external
sampling has been relocated to lms.c

This addresses item #234.