staging: comedi: cb_pcidas: refactor 'trimpot' boardinfo

Only the pci-das1602/16 boards have an AD8402 trimpot. The rest of
the boards supported by this driver have an AD7376 trimpot.

Replace the 'enum trimpot_module' in the boardinfo with a bit-field
flag 'has_ad8402' to save a bit of space.

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>