net: dsa: felix: remove felix_info :: imdio_base

This address is only relevant for the vsc9959, which is a PCIe device
that holds its switch registers in a different PCIe BAR compared to the
registers for the internal MDIO controller.

Hide this aspect from the common felix driver and move the
pci_resource_start() call to the only place that needs it, which is in
vsc9959_mdio_bus_alloc().

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>