staging: comedi: cb_pcidas: tidy up cb_pcidas_trimpot_write()

All the boards supported by this driver have an AD7376 or AD8402
trimpot. Replace the switch () with and if () and remove the
unreachable dev_err() noise.

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>