rt2x00: Unify rt2800 WPDMA ready waiting functions.

The rt2800pci_wait_wpdma_ready and rt2800usb_wait_wpdma_ready functions are
exactly the same, so unify them into rt200lib.

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