ath9k_hw: do PA offset calibration only on longcal interval

The power detector adc offset calibration has to be done
on 4 minutes interval (longcal * pa_skip_count). But the commit
"ath9k_hw: fix a noise floor calibration related race condition"
makes the PA calibration executed more frequently beased on
nfcal_pending value. Running PAOffset calibration lesser than
longcal interval doesn't help anything and the worse part is that
it causes NF load timeouts and RX deaf conditions.

In a very noisy environment, where the distance b/w AP & station
is ~10 meter and running a downlink udp traffic with frequent
background scan causes "Timeout while waiting for nf to load:
AR_PHY_AGC_CONTROL=0x40d1a" and moves the chip into deaf state.
This issue was originaly reported in Android platform where
the network-manager application does bgscan more frequently
on AR9271 chips. (AR9285 family usb device).

Cc: stable@kernel.org
Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>