staging: comedi: check data length for INSN_CONFIG_DIGITAL_TRIG

The newly defined format for the `INSN_CONFIG_DIGITAL_TRIG`
configuration instruction expects 6 data values.  Check the length in
`check_insn_config_length()` before calling the comedi subdevice's
`insn_config` handler.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>