ath9k: set 40 Mhz rate only if hw is configured in ht40

Whenever there is a channel width change from 40 Mhz to 20 Mhz,
the hardware is reconfigured to ht20. Meantime before doing
the rate control updation, the packets are being transmitted are
selected rate with IEEE80211_TX_RC_40_MHZ_WIDTH.

While transmitting ht40 rate packets in ht20 mode is causing
baseband panic with AR9003 based chips.

==== BB update: BB status=0x02001109 ====
ath: ** BB state: wd=1 det=1 rdar=0 rOFDM=1 rCCK=1 tOFDM=0 tCCK=0 agc=2
src=0 **
ath: ** BB WD cntl: cntl1=0xffff0085 cntl2=0x00000004 **
ath: ** BB mode: BB_gen_controls=0x000033c0 **
ath: ** BB busy times: rx_clear=99%, rx_frame=0%, tx_frame=0% **
ath: ==== BB update: done ====

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