phy: realtek: usb: add the error handler for nvmem_cell_read

There are following smatch warning:
drivers/phy/realtek/phy-rtk-usb2.c:901 get_phy_data_by_efuse()
error: 'buf' dereferencing possible ERR_PTR()
drivers/phy/realtek/phy-rtk-usb2.c:942 get_phy_data_by_efuse()
error: 'buf' dereferencing possible ERR_PTR()

drivers/phy/realtek/phy-rtk-usb3.c:460
get_phy_data_by_efuse() error: 'buf' dereferencing possible ERR_PTR()

The nvmem_cell_read may fail to read. So, driver must handle failure cases.

Fixes: 134e6d25 ("phy: realtek: usb: Add driver for the Realtek SoC USB 2.0 PHY")
Fixes: adda6e82 ("phy: realtek: usb: Add driver for the Realtek SoC USB 3.0 PHY")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-phy/e7ff2870-c30c-4d8d-a7a9-d2d6a4962eb5@kadam.mountain/Signed-off-by: Stanley Chang <stanley_chang@realtek.com>
Link: https://lore.kernel.org/r/20230801071509.20096-1-stanley_chang@realtek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/phy/realtek/phy-rtk-usb2.c

@@ -898,9 +898,10 @@ static int get_phy_data_by_efuse(struct rtk_phy *rtk_phy,
 		size_t buf_size;
 
 		buf = nvmem_cell_read(cell, &buf_size);
-		value = buf[0] & phy_cfg->dc_driving_mask;
-
-		kfree(buf);
+		if (!IS_ERR(buf)) {
+			value = buf[0] & phy_cfg->dc_driving_mask;
+			kfree(buf);
+		}
 		nvmem_cell_put(cell);
 	}
 
@@ -939,9 +940,10 @@ static int get_phy_data_by_efuse(struct rtk_phy *rtk_phy,
 		size_t buf_size;
 
 		buf = nvmem_cell_read(cell, &buf_size);
-		value = buf[0] & phy_cfg->dc_disconnect_mask;
-
-		kfree(buf);
+		if (!IS_ERR(buf)) {
+			value = buf[0] & phy_cfg->dc_disconnect_mask;
+			kfree(buf);
+		}
 		nvmem_cell_put(cell);
 	}
 

drivers/phy/realtek/phy-rtk-usb3.c

@@ -457,9 +457,10 @@ static int get_phy_data_by_efuse(struct rtk_phy *rtk_phy,
 		size_t buf_size;
 
 		buf = nvmem_cell_read(cell, &buf_size);
-		value = buf[0] & USB_U3_TX_LFPS_SWING_TRIM_MASK;
-
-		kfree(buf);
+		if (!IS_ERR(buf)) {
+			value = buf[0] & USB_U3_TX_LFPS_SWING_TRIM_MASK;
+			kfree(buf);
+		}
 		nvmem_cell_put(cell);
 	}