rt2x00: Simplify intf->delayed_flags locking

Instead of protecting delayed_flags with a spinlock use atomic bitops to
make the code more readable.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>