net: dwmac-sun8i: fix a missing check of of_get_phy_mode

of_get_phy_mode may fail and return a negative error code;
the fix checks the return value of of_get_phy_mode and
returns -EINVAL of it fails.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>