usb: gadget: r8a66597-udc: use devm_kzalloc() to allocate driver state

Update driver to use devm_kzalloc() to make tracking of resources
easier. Also remove the exit point via cleanup as there's no
cleanup necessary from this point now.

As a note, also removes the error print as the allocation calls
produce errors if they do not return memory.

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