ath5k: Linear PCDAC code fixes

* Set correct xpd curve indices for high/low gain curves during
   rfbuffer setup on RF5112B with both calibration curves available.

 * Don't return zero min power when we have the same pcdac value
   twice because it breaks interpolation. Instead return the right
   x barrier as we do when we have equal power levels for 2 different
   pcdac values.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>