phy: stm32: use NULL instead of zero

devm_clk_get() and devm_reset_control_get() expect a const char *id for
the last arg, but a value of zero was provided. This results in below
sparse warning:

drivers/phy/st/phy-stm32-usbphyc.c:330:42: warning: Using plain integer as NULL pointer
drivers/phy/st/phy-stm32-usbphyc.c:343:52: warning: Using plain integer as NULL pointer

Instead of zero, use NULL

Reviewed-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20200708132809.265967-6-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/phy/st/phy-stm32-usbphyc.c

@@ -327,7 +327,7 @@ static int stm32_usbphyc_probe(struct platform_device *pdev)
 	if (IS_ERR(usbphyc->base))
 		return PTR_ERR(usbphyc->base);
 
-	usbphyc->clk = devm_clk_get(dev, 0);
+	usbphyc->clk = devm_clk_get(dev, NULL);
 	if (IS_ERR(usbphyc->clk)) {
 		ret = PTR_ERR(usbphyc->clk);
 		dev_err(dev, "clk get failed: %d\n", ret);
@@ -340,7 +340,7 @@ static int stm32_usbphyc_probe(struct platform_device *pdev)
 		return ret;
 	}
 
-	usbphyc->rst = devm_reset_control_get(dev, 0);
+	usbphyc->rst = devm_reset_control_get(dev, NULL);
 	if (!IS_ERR(usbphyc->rst)) {
 		reset_control_assert(usbphyc->rst);
 		udelay(2);