rt2x00: Introduce extra queue entry sanity flag

Add a queue entry flag ENTRY_DATA_STATUS_PENDING,
which can be used to indicate a queue entry has
returned from the hardware and is waiting for
status processing. Using this flag we can add
some extra sanity checks to prevent queue corruption.

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>