rt2x00: Move packet filter flags

The packet filter flags don't belong in the interface structure
because they are device based instead of interface based.
So move the filter fields out of struct interface and into rt2x00_dev.

Additionally we shouldn't change the filter based on the working
mode, if such a thing is needed than mac80211 should have done that.

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