usb: dwc3: ep0: add dwc3_ep0_prepare_one_trb()

usb: dwc3: ep0: add dwc3_ep0_prepare_one_trb()

commit 7931ec86c1b738e4e90e58c6d95e5f720d45ee56 upstream.

For now this is just a cleanup patch, no functional
changes. We will be using the new function to fix a
bug introduced long ago by commit 0416e494ce7d
("usb: dwc3: ep0: correct cache sync issue in case
of ep0_bounced") and further worsened by commit
c0bd5456a470 ("usb: dwc3: ep0: handle non maxpacket
aligned transfers > 512")

Reported-by: Janusz Dziedzic <januszx.dziedzic@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>