staging: comedi: ni_mio_common: refactor 'num_p0_dio_channels' boardinfo

All of the board supported by this driver have at least 8 Digital I/O
channels. A couple of the PCI boards in ni_pcimio have 32 channels.

For aesthetics, change this member of the boardinfo into a bit-field
flag, 'has_32dio_chan', and use that when initializing the DIO subdevice
to set the number of channels to 32 or 8.

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>