net: dwc_eth_qos: Change eqos_ops function to static

This patch solves many warnings when compiling with W=1:
warning: no previous prototype for '....' [-Wmissing-prototypes]

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-By: Ramon Fried <rfried.dev@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>