input: raspberrypi-ts: Release firmware handle when not needed

There is no use for the firmware interface after getting the touch
buffer address, so release it.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>