net: stmmac: remove unused get_addr() callback

The last caller of the stmmac_desc_ops::get_addr() callback was removed
a while ago, so remove the unused callback.

Note that the callback also only gets half the descriptor address on
systems with 64-bit descriptor addresses, so that should be fixed if it
needs to be resurrected later.

Fixes: ec222003 ("net: stmmac: Prepare to add Split Header support")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>