net: sun8i-emac: fix printing NULL character

If the variant is not set and therefore NULL, do not attempt to print
the variant.

Signed-off-by: Lothar Felten <lothar.felten@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@openedev.com>