ath5k: Fix I/Q calibration

The sign of correction coefficients was lost in the calculations, which
caused high packetloss in 802.11a mode after the results were applied.
Fixed by removing unneccesary and broken AND with a bit mask.

Signed-off-by: Lukas Turek <8an@praha12.net>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>