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

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