ar9170: kill duplicated HT feature flag

This patch removes a redundant flag.

.cap	=	IEEE80211_HT_CAP_MAX_AMSDU |	\
>		IEEE80211_HT_CAP_SM_PS |			\ <
		[...]									\
>		IEEE80211_HT_CAP_SM_PS,			\ <
		.ampdu_factor   = 3,					\

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>