net: qcom/emac: Encapsulate sgmii ops under one structure

This patch introduces ops structure for sgmii, This by ensures that
we do not need dummy functions in case of emulation platforms.

Signed-off-by: Hemanth Puranik <hpuranik@codeaurora.org>
Acked-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>