net: lan743x: add generic implementation for phy interface selection

Add logic to read the Phy interface from MAC_CR register for LAN743x
driver.

Checks for the LAN7430/31 or pci11x1x devices and the adapter
interface is updated accordingly. For LAN7431, adapter interface is set
based on Bit 19 of MAC_CR register as MII or RGMII which removes the
forced RGMII/GMII configurations in lan743x_phy_open().

Signed-off-by: Pavithra Sathyanarayanan <Pavithra.Sathyanarayanan@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>