net: bcmgenet: refactor phy mode configuration

The DT phy mode is similar to what we want for ACPI
lets factor it out of the of path, and change the
of_ call to device_.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>