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>

drivers/staging/comedi/drivers/ni_mio_cs.c

@@ -251,7 +251,7 @@ static void mio_cs_config(struct pcmcia_device *link);
 static void cs_release(struct pcmcia_device *link);
 static void cs_detach(struct pcmcia_device *);
 
-static struct pcmcia_device *cur_dev = NULL;
+static struct pcmcia_device *cur_dev;
 
 static int cs_attach(struct pcmcia_device *link)
 {