rt2x00: Fix tx status handling in rt2800pci

The patches "rt2x00: Improve TX status entry validation" and "rt2x00: rework tx
status handling in rt2800pci" together were causing problems with tx status
processing in rt2800pci:

phy1 -> rt2800pci_txdone: Warning - Got TX status for an empty queue 3, dropping
phy1 -> rt2800pci_txdone: Warning - Got TX status for an unavailable queue 7, dropping

Fix this by using the correct field definition for getting the QID out of the
tx status report.

Reported-by: Luis Correia <luis.f.correia@gmail.com>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Tested-by: Luis Correia <luis.f.correia@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>