Staging: comedi: proc: Change file permission to read only

As there's no write operation, change to read only.
Was inadvertantly switched to 0644 in commit 1f817b86
("comedi: Don't use create_proc_read_entry()").

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>