ath9k_hw: split calib code by hardware families

Calibration code touches phy registers and since these
change the calibration code needs to be abstracted.

Noise floor calibration is the only thing remaining but
since the remaining calls only touch the AR_PHY_AGC_CONTROL
register we'll just define that register conditionally, that
will be done separately. The goal is to remove the dependency
of ar9002_phy.h on calib.c

This also adds stubs to be filled for AR9003 calibration code.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>