rt2x00: Merge RX and TX entry private data

With the pending removal of the tx_control structure
we can merge the RX and TX entry private data structure
in advance. This will temporarily increase the required
memory for the queue, but that overhead will only be limited.

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