staging: sm750fb: Add const qualifier to struct declaration.

Const qualifier added to struct declaration to avoid data overwrite
during runtime and improve security.

Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>