xhci: xhci_mem_cleanup: make sure cmd_ring_reserved_trbs really is 0

cmd_ring_reserved_trbs gets decremented by xhci_free_stream_info(), so set it
to 0 after freeing all rings, otherwise it wraps around to a very large value
when rings with streams are free-ed.

Before this patch the wrap-around could be triggered when xhci_resume
calls xhci_mem_cleanup if the controller resume fails.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>