net: cpsw: catch of_get_phy_mode failing and propagate error

It's wrong if the device tree doesn't provide a phy-mode property for
the cpsw slaves as it is documented to be required in
Documentation/devicetree/bindings/net/cpsw.txt.

Anyhow it's nice to catch that problem, still more as it used to work
without this property up to commit 388367a5 (drivers: net: cpsw: use
cpsw-phy-sel driver to configure phy mode) which is in v3.13-rc1.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>