vmod:[fix] lun_to_dev() must check all device instances

Problem:
if you declare the first vmod module with LUN=2 (or any number grater than 0)
then the driver will not load the mezzanines.

The real device instance is stored in device_table according to its LUN.
This means that we have to loop over all the table to find the correct
instance. Before the condition was the device instances counter.

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>

vmod/driver/vmodio.c

@@ -79,7 +79,7 @@ static struct vmodio *lun_to_dev(int lun)
 {
 	int i = 0;
 
-	for (i = 0; i < devices; i++) {
+	for (i = 0; i < MAX_DEVICES; i++) {
 		struct vmodio *dev = &device_table[i];
 		if (dev->lun == lun)
 			return dev;