usbnet: changes for upcoming cdc_ncm driver

Changes:
include/linux/usb/usbnet.h:
- a new flag to indicate driver's capability to accumulate IP packets in Tx
 direction and extract several packets from single skb in Rx direction.
drivers/net/usb/usbnet.c:
- the procedure of counting packets in usbnet was updated due to the
 accumulating of IP packets in the driver
- no short packets are sent if indicated by the flag in driver_info
 structure

Signed-off-by: Alexey Orishko <alexey.orishko@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>