ath9k_hw: Fix hw reset failure with HTC driver

The following commit removed DISABLE_REGWRITE_BUFFER ops. The unnecessary
REGWRITE_BUFFER_FLUSH was not removed properly which is causing failure on
hw reset.

Author: Felix Fietkau <nbd@openwrt.org>
Date:   Tue Oct 5 12:03:42 2010 +0200

    ath9k_hw: clean up register write buffering

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>