ath9k: disable the tasklet before taking the PCU lock

Fixes a reported CPU soft lockup where the tasklet tries to acquire the
lock and blocks while ath_prepare_reset (holding the lock) waits for it
to complete.

Cc: stable@vger.kernel.org
Reported-by: Robert Shade <robert.shade@gmail.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>