ath9k: always use SIFS times from OFDM for 5/10 MHz

5/10 MHz channels should always use SIFS times as defined in IEEE
802.11-2012 18.4.4 (OFDM PHY characteristics). This makes it compatible
to ath5k, which does the same.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>