net: ethernet: emac: Fix phy mode type

Pass a phy_interface_t to of_get_phy_mode(), by changing the type of
phy_mode in the device structure. This then requires that
zmii_attach() is also changes, since it takes a pointer to phy_mode.

Fixes: 0c65b2b9 ("net: of_get_phy_mode: Change API to solve int/unit warnings")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>