ath9k_hw: check GPM HW write pointer before chip reset

Both "MAC Warm Reset" and "MCI Reset Rx" will reset GPM HW write_ptr.
We should check software cached write_ptr against HW write_ptr before
reset. Otherwise the pending DMA data will be lost.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>