staging: comedi: cb_pcidas: clarify the ai cmd->start_arg validation

This driver supports two cmd->start_src values, TRIG_NOW and TRIG_EXT.

For aesthetics, replace the default case when validating the cmd->start_arg
with the proper case statement.

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>