Add support for updating firmware, triggered by a sysfs attribute.

This patch has been tested on Synaptics S7300.
Signed-off-by: Nick Dyer <nick@shmanahar.org>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>