usb: wusbcore: add a convenience function for completing a transfer segment

This patch adds a convenience function for the commonly performed task
of marking a transfer segment as done.  It combines the 3 steps of
setting the segment status, incrementing the segs_done field of the
transfer and checking if the completed segment results in the transfer
also being done.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>