staging: comedi: ni_labpc_common: use CMDF_PRIORITY

Replace use of `TRIG_RT` command flag with the equivalent flag
`CMDF_PRIORITY`.  The numeric value is unchanged.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>