net: lan743x: Use correct variable in lan743x_sgmii_config()

There is a copy and paste bug in lan743x_sgmii_config() so it checks
if (ret < 0) instead of if (mii_ctl < 0).

Fixes: 46b777ad ("net: lan743x: Add support to SGMII 1G and 2.5G")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/YrRry7K66BzKezl8@kili


Signed-off-by: Jakub Kicinski <kuba@kernel.org>