ravb: Factorise ravb_adjust_link function

R-Car supports 100 and 1000 Mbps transfer speed whereas RZ/G2L
in addition support 10Mbps. Factorise ravb_adjust_link function
in order to support 10Mbps speed.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>