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

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>

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

@@ -315,6 +315,10 @@ static void qusb2_phy_set_tune2_param(struct qusb2_phy *qphy)
 	const struct qusb2_phy_cfg *cfg = qphy->cfg;
 	u8 *val;
 
+	/* efuse register is optional */
+	if (!qphy->cell)
+		return;
+
 	/*
 	 * Read efuse register having TUNE2/1 parameter's high nibble.
 	 * If efuse register shows value as 0x0, or if we fail to find