rt2x00: Make queue_entry flags access atomic

All access to the queue_entry->flags can be done concurrently,
so all flags must use the atomic operators. On most locations
this was already done, so just fix the last few non-atomic
versions.

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