rt2x00: Move init_txring and init_rxring into rt2x00lib

Prior to enabling the radio rt2x00lib should go through all
rings and for each entry should call the callback function
init_txentry() and init_rxentry().

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