net: add helper eth_hw_addr_crc

Several drivers use the same code as basis for filter hashes. Therefore
let's factor it out to a helper. This way drivers don't have to access
struct netdev_hw_addr internals.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>