mwifiex: do not process broadcast mac address for del_sta

Generally broadcast mac address deauth is followed by stop_ap or start_ap.
In both cases, FW already has provision to send deauth; so there is no
need to handle broadcast mac deauthentication.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>