net: smc911x: Determine bus width at runtime

The SMC911x Ethernet MACs can be integrated using a 16 or 32-bit bus.
The driver needs to know about this choice, which is the reason for us
having a Kconfig symbol for that.

Now this bus width is already described using a devicetree property, and
since the driver is DM compliant and is using the DT now, we should query
this at runtime. We leave the Kconfig choice around, in case the DT is
missing this property.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>