usb: dwc3: gadget: split __dwc3_gadget_kick_transfer()

To aid code readability, we're gonna split
__dwc3_gadget_kick_transfer() into its constituent
parts: scatter gather and linear buffers.

That way, it's easier to follow the code and focus
debug effort when one or the other fails.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>