usb: gadget: r8a66597-udc: cleanup error path

With the updates for devm, the cleanup path no longer needs to
check for NULL device state, so remove it and return directly
if the irq resource missing

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Felipe Balbi <balbi@ti.com>