ath9k: do not miss longcal on AR9002

Each of AGC & I/Q calibrations can take a long time. Long calibration
and NF calibration in particular are forbiden for parallel run with
ADC & I/Q calibrations. So, the chip could not be ready to perform the
long calibration at the time of request. And a request to perform the
long calibration may be lost.

In order to fix this, preserve the long calibration request as a
calibration state flag and restore the long calibration request each
time the calibration function is called again (i.e. on each subsequent
ivocation of the short calibration).

This feature will be twice useful after the next change, which will
make it possible to start the long calibration before all ADCs & I/Q
calibrations are completed.

Run tested with AR9220.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200424004923.17129-4-ryazanov.s.a@gmail.com