usb: gadget: dummy_hcd: move the transfer part into its own function

This patch moves the part of the code which does the bare transfer into
its function. It is a preparion for the implementation of sg support.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>