ath5k: Use correct value for min_pwr and cur_pwr

Make sure we don't store the table offsets for min and cur power levels,
store the 0.25dB values instead. This way we don't clamp the tx power level
to max (because now cur_pwr holds the 0.25dB value, not the table offset) after
re-using cur_pwr on reset.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>