usb: dwc3: Add support to force speed to ssp

Currently only super and high speed can be forced. Add support to
force super speed plus speed (ssp) so that speed can be switched between
high to super to ssp in any order.

Change-Id: Ie675e39fb577282c4eba88946a40ca54046692b2
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>