ath9k_hw: remove bogus register write on ath9k_hw_9271_pa_cal()

An extra register was being written to for PA calibration
making the hardware unresponsive, remove it. Hardware
reset should now complete fine on ar9271.

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