ath5k: move noise floor calibration into tasklet

Seperate noise floor calibration from other PHY calibration and move it to the
tasklet. This is the first step to more separation of different calibrations.

Also move out ath5k_hw_request_rfgain_probe(ah) so we have one clean function
for I/Q calibration on 5111x parts.

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