ath5k: Update phy calibration functions

 * Enable I/Q calibration each time we have correction results (we
   were only enabling calibration during reset). If we don't we commit
   the same results each time calibration routine is called.
 * Add some documentation and a TODO on nf calibration
 * Return -EAGAIN on noise floor timeout/failure

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