net: mvpp2: move the mac retrieval/copy logic into its own function

The MAC retrieval has a quite complicated logic (which is broken). Moves
it to its own function to prepare for patches fixing its logic, so that
reviews are easier.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>