Clang static analysis reports this problem
mtk_eth_soc.c:394:7: warning: Branch condition evaluates
  to a garbage value
                if (err)
                    ^~~

err is not initialized and only conditionally set.
So intitialize err.

Fixes: 7e538372 ("net: ethernet: mediatek: Re-add support SGMII")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>