usb: gadget: r8a66597-udc: use devm_request_irq() to get device irq

Use the devm_request_irq() call to get the interrupt for the
device and have it automatically free on exit.

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