brcm80211: smac: changed check to confirm STA only support

The driver currently only supports STA operation. However, in
brcms_ops_add_interface() also AP and SSID mode were accepted.

Reviewed-by: Alwin Beukers <alwin@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>