USB: dwc3: use device_{has,get}_property()

Switch the driver to use device based functions which will work not
only with FDT but also ACPI.

While here make dr_mode a local variable as it is only used during
probe and not needed later in the softc.

MFC after: 2 weeks
Reviewed by: mw
Differential Revision: https://reviews.freebsd.org/D33170