Pull up following revision(s) (requested by jdolecek in ticket #787):

sys/dev/usb/xhci.c: revision 1.88-1.90
sys/dev/usb/xhcireg.h: revision 1.10

add KASSERT() that sc_child* is set to NULL after child detach; just for
readability, it's not immediatelly obvious this is done in xhci_childdet()
no functional changes

trigger the softint processing on that child bus which is not detached yet
fixes PR kern/53066 by Martin Husemann

enable code to only trigger usb processing when EINT is set, to
avoid misinterpreting shared interrupt for another device

when clearing USBSTS, actually preserve the bits which spec requires to
preserve, and actually clear bit 1, which should be actually always
cleared to zero by spec

also #ifdef XHCI_DEBUG some unnecessary register reads
this should finally resolve PR kern/53066 also for Martin

(martin)