video: fbdev: add const to bin_attribute structures

Add const to bin_attribute structures as they are only passed to the
functions sysfs_{remove/create}_bin_file or
device_{remove/create}_bin_file. The corresponding arguments are of
type const, so declare the structures to be const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>