rt2x00: Extend PLCP descriptor definition for rt2400pci

Extend word field definitions for the PLCP words
in the TX descriptor to contain the BBP fields as well.
This will remove rt2400pci_write_tx_desc() from the
checkstack script.

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