Use a synchronous usb_bulk_msg() when switching link speed in
set_termios(). This way we do not need to keep track of outstanding URBs
in order to be able to stop them at close.

Note that there's no need to set URB_ZERO_PACKET as the one-byte
transfer will always be short.
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>