Input: ati_remote2 - add per device attrs

Add per device channel_mask and mode_mask attributes. They inherit
the values from the module parameters when the device is registered.
One additional benefit is that now runtime changes to channel_mask
can actually affect the hardware channel setup like they should.

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