ath9k_htc: Fix disconnect issue in HT40 mode.

Some APs advertise that they may be HT40 capable in the capabilites
but the current operating channel configuration may be only HT20.
This causes disconnection as ath9k_htc sets WLAN_RC_40_FLAG despite
the AP operating in HT20 mode.
Hence set this flag only if the current channel configuration
is HT40 enabled.

Cc: stable@kernel.org
Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>