staging: comedi: ni_stc.h: add a 'is_m_series' flag to private data

The (board->reg_type & ni_reg_m_series_mask) test is commonly used
in ni_mio_common.c to detect if the driver is being used with a
National Instruments "M series" DAQ board. Simplify the code a bit
by adding an 'is_m_series' bit-field flag to the private data.

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