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

drivers/phy/sunplus/phy-sunplus-usb2.c

@@ -92,13 +92,13 @@ static int update_disc_vol(struct sp_usbphy *usbphy)
 	otp_v = nvmem_cell_read(cell, &otp_l);
 	nvmem_cell_put(cell);
 
-	if (otp_v) {
+	if (!IS_ERR(otp_v)) {
 		set = *(otp_v + 1);
 		set = (set << (sizeof(char) * 8)) | *otp_v;
 		set = (set >> usbphy->disc_vol_addr_off) & J_DISC;
 	}
 
-	if (!otp_v || set == 0)
+	if (IS_ERR(otp_v) || set == 0)
 		set = OTP_DISC_LEVEL_DEFAULT;
 
 	val = readl(usbphy->phy_regs + CONFIG7);