usb: gadget: atmel_usba_udc: convert to newstyle 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 the_udc
variable in start/stop functions. I kept the the_udc variable since it
makes the init code a little simpler.
Someone with hardware might want to look if it possible to move the vbus
irq/toggle_bias code into ->pullup().

Compile tested only.

Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Felipe Balbi <balbi@ti.com>