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>