net: clean up usbnet_receive()

net: clean up usbnet_receive()

The USB network interface has two code paths depending on whether or not
RNDIS mode is enabled.  Refactor usbnet_receive() so that there is a
common path throughout the function instead of duplicating everything
across if (is_rndis(s)) ... else ... code paths.

Clean up coding style and 80 character line wrap along the way.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
(cherry picked from commit f237ddbb89142c6948a2257c459e49dee7500a7c)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>