rt2x00: Improve TX status entry validation

rt2x00: Improve TX status entry validation

The TX_STA_FIFO contains some information for identifying
a outgoing frame, however matching by WCID and ACK status is
not sufficient to 100% identify the macthing queue_entry structure
(containing the SKB buffer) which belongs to the status report.

Within TX_STA_FIFO we have a 4-bit field named PACKETID, which is
currently used to encode the queue id. The queue ID is however
limited to values from 0 to 3, which means 2 bits are sufficient
to encode the value. With the remaining 2 bits we can encode a
partial queue_entry index number. The value of PACKETID is not
allowed to become 0, with the queue ID ranging from 0 to 3, at least
one of the bits for the entry identification must be 1.

That leaves us with 3 possible values we can still encode in the
bits. Altough this doesn't allow 100% accurate matching of the
TX_STA_FIFO queue to a queue_entry structure, it at least improves
the accuracy. This allows us to better detect if we have missed the
TX_STA_FIFO report, which in turn reduces the number of watchdog
warnings regarding the TX status timeout.

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