net: cdc_ncm/cdc_mbim: adding NCM protocol statistics

To have an idea of the effects of the protocol coalescing
it's useful to have some counters showing the different
aspects.

Due to the asymmetrical usbnet interface the netdev
rx_bytes counter has been counting real received payload,
while the tx_bytes counter has included the NCM/MBIM
framing overhead. This overhead can be many times the
payload because of the aggressive padding strategy of
this driver, and will vary a lot depending on device
and traffic.

With very few exceptions, users are only interested in
the payload size.  Having an somewhat accurate payload
byte counter is particularly important for mobile
broadband devices, which many NCM devices and of course
all MBIM devices are. Users and userspace applications
will use this counter to monitor account quotas.

Having protocol specific counters for the overhead, we are
now able to correct the tx_bytes netdev counter so that
it shows the real payload

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>