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>