staging: comedi: das16m1: rename struct das16m1_private_struct

For aesthetics, remove the redundant '_struct' suffix.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>