Input: sgi_btns - switch to using managed resources

Switching to devm API allows to clean up error handling paths and drop
the remove() method.

Link: https://lore.kernel.org/r/20191017204217.106453-14-dmitry.torokhov@gmail.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>