staging: comedi: ni_mio_common: fix subdevice flags for PFI subdevice

The PFI subdevice flags indicate that the subdevice is readable and
writeable, but that is only true for the supported "M-series" boards,
not the older "E-series" boards.  Only set the SDF_READABLE and
SDF_WRITABLE subdevice flags for the M-series boards.  These two flags
are mainly for informational purposes.

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