net: qcom/emac: fix device tree initialization

Commit "net: qcom/emac: Encapsulate sgmii ops under one structure"
introduced the sgmii_ops structure, but did not correctly initialize
it on device tree platforms.  This resulted in compiler warnings when
ACPI is not enabled.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>