net: fman: fix variable type

commit  9cc41a972bcce5b271e817a2fe9684ec3913dfac from
https://github.com/nxp-imx/linux-imx lf-6.1.y

fwnode_get_phy_mode returns an int, not an unsigned value. Fix phy_if's
type.

Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>