staging: comedi: ni_mio_common: remove BUG() check in ni_cdio_cmd()

The cmd->scan_begin_src was validated in ni_cdio_cmdtest() and can only
be TRIG_EXT. Remove the switch statement and unnecessary BUG() check.

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>