net: mhi: Improve MBIM packet counting

Packets are aggregated over the MBIM link and currently the MHI net
device will count each aggregated packet rather then the actual
packets themselves.

If a protocol handler module is specified, use that to count the
packets rather than directly in the MHI net device. This is in line
with the behaviour of the USB net cdc_mbim driver.

Signed-off-by: Richard Laing <richard.laing@alliedtelesis.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>