usb: host: xhci: convert to list_for_each_entry_safe()

instead of using while(!list_empty()) followed by list_first_entry(), we
can actually use list_for_each_entry_safe().

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>