usbnet: Fix up 'FLAG_POINTTOPOINT' and 'FLAG_MULTI_PACKET' overlaps.

USB tethering does not work anymore since 2.6.39-rc2, but it's okay in
-rc1. The root cause is the new added mask code 'FLAG_POINTTOPOINT'
overlaps 'FLAG_MULTI_PACKET'  in  include/linux/usb/usbnet.h, this
causes logic issue in  rx_process(). This patch cleans up the overlap.

Reported-and-Tested-by: Gottfried Haider <gottfried.haider@gmail.com>
Signed-off-by: Huajun Li <huajun.li.lee@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>