bhyve: Drop volatile qualifiers from xhci hw struct fields

This fixes a warning raised by the removal of the volatile qualifier
from &trb->qwTrb0 in the following snippet:

xfer_block = usb_data_xfer_append(xfer,

	     (void *)(trbflags & XHCI_TRB_3_IDT_BIT ?
		 &trb->qwTrb0 : XHCI_GADDR(sc, trb->qwTrb0)),
	     trb->dwTrb2 & 0x1FFFF, (void *)addr, ccs);

The use of volatile appears to be inherited from the kernel driver's
definitions of the same structures. It makes some sense, since USB TRBs
and related structures live in guest memory, but bhyve device models
generally don't volatile-qualify accesses to guest memory and I can't
see how they are required for correctness here. Moreover, XHCI_GADDR
does not return volatile pointers so we're already being inconsistent.
Just drop the qualifiers to address the warning.

MFC after: 2 weeks
Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D37290