net: ethernet: mtk_eth_soc: fix error checking in mtk_mac_config()

commit 214b3369ab9b0a6f28d6c970220c209417edbc65 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>