rt2x00: Don't use queue entry as parameter when creating TX descriptor.

The functions that create the tx descriptor structure do not operate on
a queue entry at all. Signal this fact in the code by not providing a
queue entry as a parameter, but the rt2x00 device structure and the skb
directly.

This patch is a preparation for reducing the time a queue is locked for
a tx operation.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>