rt2x00: TO_DS filter depends on intf_ap_count

The TO_DS filter does not only depend on the FIF_PROMISC_IN_BSS flag
provided by mac80211, but also on the intf_ap_count count.
This makes sense, since when Master mode is active, we should all frames
that are send to the active AP (the device itself).

This means that when an interface is added we should force the
packet filter to be updated during the next mac80211 call of
configure_filter() to make sure the intf_ap_count field is checked.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>