net: ll_temac: Fix an NULL vs IS_ERR() check in temac_open()

The phy_connect() function doesn't return NULL pointers.  It returns
error pointers on error, so I have updated the check.

Fixes: 8425c41d ("net: ll_temac: Extend support to non-device-tree platforms")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>