siox: fix possible buffer overflow in device_add_store

commit f87deada upstream.

Width 20 given in format string is larger than destination
buffer 'type[20]', use %19s to prevent overflowing it.

Fixes: bbecb07f ("siox: new driver framework for eckelmann SIOX")
Cc: stable <stable@vger.kernel.org>
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Gavin Schenk <g.schenk@eckelmann.de>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>