ravb: Factorise ravb_emac_init function

The E-MAC IP on the R-Car AVB module has different initialization
parameters for RX frame size, duplex settings, different offset
for transfer speed setting and has magic packet detection support
compared to E-MAC on RZ/G2L Gigabit Ethernet module. Factorise
the ravb_emac_init function to support the later SoC.

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>