rt2x00: Move filling of TX URB to rt2x00usb_kick_tx_entry function.

There is no need to fill the TX URB this early, and moving it to the
rt2x00usb_kick_tx_entry function allows us to merge the PCI and USB
variants of the write_tx_data function.

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>