usb: host: xhci: remove incorrect comment about mutex

The comment stating that xhci_setup_device() is protected by the address
mutex is not true since
commit 6fecd4f2

 ("USB: separate usb_address0 mutexes for each bus")
as xhci handles two buses.

Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>