net: mvneta: Fix fault when wrong device tree

The driver does not check id phy_connect failed (for example on wrong
property name in device tree). In such a case a fault occurs and the
CPU is restarted.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>