rt2x00: Fix check for BSS info changes

Fix ERP configuration, due to a too strict changes
flags checking we never updated the short slot time
or basic rate mask when no other changes were made
at the same time.

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