dwc3: improve debugging

Rather than hiding behind #if 0, hide the debugging behind DWC3_DEBUG
so it can be turned on with a single define. Require bootverbose
to print anything so we can still avoid spamming the console if DWC3_DEBUG
is on.
Harmonize the format string in snsp_dwc3_dump_regs() to always print the
full register and also print the XHCI quirks.
Call snsp_dwc3_dump_regs() twice, before and after generic XHCI attachment
and initialisation as this may have an effect on the confirgumation state.

Obtained from: an old debug patch
MFC after: 2 weeks
Reviewed by: mw
Differential Revision: https://reviews.freebsd.org/D35700