ARM: stm32: Perform node compatible check for KS8851 early

Check the compatible string of ethernet1 node for KS8851 very early on,
before calling uclass_get_device_by_of_path() which might initialize
the device and possibly attempt to configure MAC address into device
which is not KS8851. Doing the compatibility check early prevent this.

Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>