mwl8k: Set hardware flag IEEE80211_HW_AP_LINK_PS

This will avoid mac80211 to trigger PS mode for connected station
based on the PM bit of incoming frames. AP firmware is capable of
handling such frames and buffering TX frames destined to the
stations that are in PS mode.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>