phy: Sparx5 Eth SerDes: Fix return value check in sparx5_serdes_probe()

phy: Sparx5 Eth SerDes: Fix return value check in sparx5_serdes_probe()

In case of error, the function devm_ioremap() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value
check should be replaced with NULL test.

Fixes: 2ff8a1eeb5aa ("phy: Add Sparx5 ethernet serdes PHY driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20210909072149.2934047-1-yangyingliang@huawei.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>