eth_type_trans() currently pulls dev->hard_header_len off a frame
passed to it, however always interpreting it as a ethernet header.

Grepping shows that it is only used on net devices where
dev->hard_header_len == ETH_HLEN. It makes more sense to actually
pull of ETH_HLEN for the header (it's treated as a struct of the length
anyway), not changing the behavior for the existing users but allowing
two places which had to use their private copies of eth_trans_type to
use the generic routine now.

One place is in drivers/net/hamachi.c and converted in this cset, the other
one is in the ISDN network code, patch will follow.