ath9k: Fix bug in determining HT40 mode

This patch fixes a bug in checking for HT40 mode.
The STA's mode can be determined from the cached HT capabilities,
use that and remove the redundant variable 'rc_phy_mode'.

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