rt2x00: Move driver callback functions into the ops structure

All callback functions are gathered in rt2x00dev->ops except
for the callback functions which are used in rt2800lib to
acces rt2800pci/usb.

Move the priv pointer from rt2x00dev to rt2x00dev->ops and
rename it to drv to make it obvious that it is the driver callback
structure.

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