staging/comedi: fix the initialize statics issue in drivers/ni_mio_cs.c

fixed below checkpatch error.
- ERROR: do not initialise statics to 0 or NULL

Signed-off-by: YAMANE Toshiaki <yamanetoshi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>