usb: gadget: at91_udc: convert to new style start/stop interface

This patches converts the driver into the new style start/stop interface.
As a result the driver no longer uses the static global controller
variable in start/stop functions. I kept the controller variable since it
makes the init code a little simpler.
Compile tested only.

Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Felipe Balbi <balbi@ti.com>