ath5k: Calibration re-work

Noise floor calibration does not interfere with traffic and should run more
often as part of our "short calibration". The full calibration is not the
noise floor calibration but the AGC + Gain_F (on RF5111 and RF5112) calibration
and should run less often because it does interfere with traffic.

So

Short calibration -> I/Q & NF Calibration
Long calibration -> Short + AGC + Gain_F

This patch was for some time on my pub/ dir on www.kernel.org and has been tested
by a few people and me. I think it's O.K. to go in.

I also changed ah_calibration to ah_iq_cal_needed to make more sense.

v2 Use a workqueue instead of a tasklet for calibration

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