rt2x00: Add rt2x00dev->flags to debugfs

Loosely based on the patch by Matthijs Kooijman,
this will add the dev_flags entry into debugfs which
will display rt2x00dev->flags.
This will allow easier debugging of flag handling.

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