rt2x00: Merge PCI and USB versions of write_tx_data into single function.

Now that rt2x00pci_write_tx_data and rt2x00usb_write_tx_data are similar
we can merge them in a single function in rt2x00queue.c.

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