rt2x00: Cleanup TX/RX entry handling

Merge the callback functions init_txentry() and
init_rxentry(). This makes life in rt2x00lib a
lot simpler and we can cleanup several functions.

rt2x00pci contained "fake" FIELD definitions for
descriptor words. This is not flexible since it
assumes the driver will always have the same field
to indicate if a driver is available or not.
This should be dependent on the driver, and we
should add a callback function for this.

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