usb: dwc3: gadget: Refactor ep command completion

Refactor END_TRANSFER command completion handling and move it outside of
the switch statement to its own function. This makes it cleaner and
consistent with other event handler functions. No functional change
here.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>