phy: usb: sunplus: Fix return value check in update_disc_vol()

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

Fixes: 99d9ccd9 ("phy: usb: Add USB2.0 phy driver for Sunplus SP7021")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20220909094709.1790970-1-weiyongjun@huaweicloud.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>