ath5k: IQ calibration for AR5211 is slightly different

according to the HAL sources the calculation of the Q value is slightly
different for AR5211 chips.

i couldn't test this since IQ calibration never finishes on older parts. this
is a different problem...

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>