Input: ati_remote2 - return the error code from sysfs channel_mask store

If the channel_mask setup fails when configuring it through sysfs,
return the error code to the caller instead of claiming success.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>