rt2x00: Move code into seperate functions

Some functions have grown rapidly in size over the last time,
some of those functions (like the rt2x00queue_create_tx_descriptor)
will further increase in size soon, so it is best to start cutting
it into logical pieces.

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