usb: gadget: r8a66597-udc: keep dev as reference to &pdev->dev

Remove usages of &pdev->dev in the driver probe function
with just dev to make the references to it easier to
write. Convert all the current users of it to use it.

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