usb: gadget: r8a66597-udc: fix cannot connect after rmmod gadget driver

When we run rmmod a gadget driver, the driver will call
disable_controller(). Then, because the bit of USBE in SYSCFG0 was
cleared in on_chip=1 mode, we could not connect the usb when we run
insmod a gadget driver next time.
This patch also cleans up probe() and ->stop() about unnecessary
init_controller().

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>