phy: qcom-qusb2: Fix crash if nvmem cell not specified

commit 0b4555e7 upstream.

Driver currently crashes due to NULL pointer deference
while updating PHY tune register if nvmem cell is NULL.
Since, fused value for Tune1/2 register is optional,
we'd rather bail out.

Fixes: ca04d9d3 ("phy: qcom-qusb2: New driver for QUSB2 PHY on Qcom chips")
Reviewed-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Reviewed-by: Evan Green <evgreen@chromium.org>
Cc: stable <stable@vger.kernel.org> # 4.14+
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/phy/qualcomm/phy-qcom-qusb2.c

@@ -178,6 +178,10 @@ static void qusb2_phy_set_tune2_param(struct qusb2_phy *qphy)
 	struct device *dev = &qphy->phy->dev;
 	u8 *val;
 
+	/* efuse register is optional */
+	if (!qphy->cell)
+		return;
+
 	/*
 	 * Read efuse register having TUNE2 parameter's high nibble.
 	 * If efuse register shows value as 0x0, or if we fail to find