usb: dwc3: fix randconfig build errors

commit 388e5c51 (usb: dwc3: remove dwc3 dependency
on host AND gadget.) created the possibility for
host-only and peripheral-only dwc3 builds but
left a possible randconfig build error when host-only
builds are selected.

Cc: <stable@vger.kernel.org> # v3.8+
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>