ipheth: Properly distinguish length and alignment in URBs and skbs

The USB protocol this driver implements appears to require 2 bytes of
padding in front of each received packet.  This used to be equal to
the value of NET_IP_ALIGN on x86, so the driver abused that constant
and mostly worked, but this is no longer the case.  The driver also
mixed up the URB and packet lengths, resulting in 2 bytes of junk at
the end of the skb.

Introduce a private constant for the 2 bytes of padding; fix this
confusion and check for the under-length case.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>