usb: gadget: imx_udc: convert to new style start/stop

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 code. I kept the gloval controller variable because
it keeps init simple.
Compile tested only.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Felipe Balbi <balbi@ti.com>