net: mv643xx_eth: potential NULL dereference in probe()

We assume that "mp->phy" can be NULL a couple lines before the
dereference.

Fixes: 1cce16d3

 ('net: mv643xx_eth: Add missing phy_addr_set in DT mode')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: David S. Miller <davem@davemloft.net>