Pull up following revision(s) (requested by skrll in ticket #11):
	sys/dev/usb/xhci.c: revision 1.25
Serialise xhci_intr1 calls with sc_intr_lock.  From Takahiro HAYASHI.
Pull up following revision(s) (requested by skrll in ticket #12):
	sys/dev/usb/files.usb: revision 1.133
	sys/dev/usb/usb.h: revision 1.108
Add XHCI_DEBUG.  From Takahiro HAYASHI with tweaks from me.
One day someone(tm) will tidyup USB debug stuff.
Tickets 11, 12