net: dsa: xrs700x: use of_match_ptr() on xrs700x_mdio_dt_ids

Use of_match_ptr() on xrs700x_mdio_dt_ids so that NULL is substituted
when CONFIG_OF isn't defined. This will prevent unnecessary use of
xrs700x_mdio_dt_ids when CONFIG_OF isn't defined.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>