usb: dwc3: pci: Fix dr_mode misspelling

usb_get_dr_mode() expects the device-property to be spelled
"dr_mode" not "dr-mode".

Spelling it properly fixes the following warning showing up in dmesg:
[ 8704.500545] dwc3 dwc3.2.auto: Configuration mismatch. dr_mode forced to gadget

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>