usb: dwc3: dbm: Fix DBM speed configuration

Without this fix, DBM_GEN_CFG is always configured
with 0 - meaning HS.
This fix allow configuring HS/SS properly.

Change-Id: I9ce7e946cac444e4d70c22979a3c8600122acde5
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>