net: stmmac: failure to probe without MAC interface specified

Alexander Stein reports that commit a014c355

 ("net: stmmac: clarify
difference between "interface" and "phy_interface"") caused breakage,
because plat->mac_interface will never be negative. Fix this by using
the "rc" temporary variable in stmmac_probe_config_dt().

Reported-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://lore.kernel.org/r/E1qayn0-006Q8J-GE@rmk-PC.armlinux.org.uk


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