drivers: ethernet: eth_sam_gmac: Do not force RMII

The `link_configure` function currently configures the `GMAC_UR`
register and forces the RMII interface.

This is not necessary because the `GMAC_UR` register is already
configured with an appropriate value based on `CONFIG_ETH_SAM_GMAC_MII`
in `gmac_init`.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>