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>