ath5k: fix interpolation with equal power levels

When the EEPROM contains weird values for the power levels we have to
fix the interpolation process.

Signed-off-by: Fabio Rossi <rossi.f@inwind.it>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>