phy: phy-ocelot-serdes: fix return value check in serdes_probe()

In case of error, the function syscon_node_to_regmap() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Fixes: 51f6b410 ("phy: add driver for Microsemi Ocelot SerDes muxing")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>