net: mv88e6xxx: fix missing SMI address initialization

The mv88e6xxx driver does not currently initialize the smi_addr field, but
instead keeps the default zero value. This leads to driver being unusable
on devices where the switch is not on address zero of the mdio bus. Fix
this problem by reading the SMI address from device tree.

Signed-off-by: Marek Mojík <marek.mojik@nic.cz>
Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>