libbladeRF: Cleaned up LMS VGA range checks, RXVGA1 LUT and enable bit

Moved LMS VGA range checks out of bladeRF.c into lms.c.

Replaced RXVGA1 calculations with lookup tables. (h/t to LazyDodo for
the suggestion.)

Corrected RXVGA1 enable/disable. Previously we were just setting the
bias current to 0, but it turns out there's an actual bit for this in
reserved register 0x7d.