usb: dwc3: Allow dwc_usb31 to operate in dual-role

According to the comment in dwc3_get_dr_mode() controller
revisions > 3.30a, including dwc_usb31, do not support OTG.
However, dual-role switching can still be supported with the
use of an extcon or usb_role_switch driver, so there is no
need to restrict the operation to host-only or peripheral-only.

Change-Id: I7dea9ff15f5ac15f673c4569ffe401981de04f22
Signed-off-by: Jack Pham <jackp@codeaurora.org>