This is a preparatory for the next patch that autodetects the device to
write to. To not have to modify the callback data for each call, some
static functions get an additional parameter to allow that.

This doesn't affect boards making use of the file's function and doesn't
change behaviour.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>