libbladeRF: Check against VCO_LOW/VCO_HIGH.

When doing the search for the correct VCOCAP value, check to make
sure we are not equal to the edge stopping condition instead of
checking against VCO_NORM.  This avoids a problematic situation when
the found VCO_NORM value is on the cusp of being unlocked, causing
the upper bound to end prematurely.