usb: dwc3: core: avoid NULL pointer dereference

commit 3e10a2ce ("usb: dwc3: add hsphy_interface
property") introduced a possible NULL pointer
dereference because dwc->hsphy_interface can be
NULL.

In order to fix it, all we have to do is guard
strncmp() against a NULL argument.

Fixes: 3e10a2ce ("usb: dwc3: add hsphy_interface property")
Tested-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/core.c

@@ -446,10 +446,12 @@ static int dwc3_phy_setup(struct dwc3 *dwc)
 	/* Select the HS PHY interface */
 	switch (DWC3_GHWPARAMS3_HSPHY_IFC(dwc->hwparams.hwparams3)) {
 	case DWC3_GHWPARAMS3_HSPHY_IFC_UTMI_ULPI:
-		if (!strncmp(dwc->hsphy_interface, "utmi", 4)) {
+		if (dwc->hsphy_interface &&
+				!strncmp(dwc->hsphy_interface, "utmi", 4)) {
 			reg &= ~DWC3_GUSB2PHYCFG_ULPI_UTMI;
 			break;
-		} else if (!strncmp(dwc->hsphy_interface, "ulpi", 4)) {
+		} else if (dwc->hsphy_interface &&
+				!strncmp(dwc->hsphy_interface, "ulpi", 4)) {
 			reg |= DWC3_GUSB2PHYCFG_ULPI_UTMI;
 			dwc3_writel(dwc->regs, DWC3_GUSB2PHYCFG(0), reg);
 		} else {