The actual return value of eth_header() is never used, only its sign.
So it does not make a difference if we return dev->hard_header_len or
ETH_HLEN, but the latter makes more sense as that is the number of bytes
we added to the front of the frame.

For 99% of the drivers, dev->hard_header_len == ETH_HLEN, so no difference
at all, but if a driver actually needs additional headroom and thus
has dev->hard_header_len > ETH_HLEN, the process of building the ethernet
header should not care at all.