mediatek: checking for IS_ERR() instead of NULL

of_phy_connect() returns NULL on error, it never returns error pointers.

Fixes: 656e7052

 ('net-next: mediatek: add support for MT7623 ethernet')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>