net: fec_mxc: specify the registered eth index by dev_id

Specify the registered eth index by dev_id.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>