staging: comedi: das1800: absorb control_a_bits()

Absorb this helper function into the analog input (*do_cmd).
For aesthetics, convert the switch code into if/else.

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>