dpaa2-eth: fix an always true condition in dpaa2_mac_get_if_mode

Convert the phy_mode() function to return the if_mode through an
argument, similar to the new form of of_get_phy_mode().
This will help with handling errors in a common manner and also will fix
an always true condition.

Fixes: 0c65b2b9

 ("net: of_get_phy_mode: Change API to solve int/unit warnings")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>