drivers/sensors/lis2mdl: Fix SYSLOG call.

This call to syslog was missing the '%' in its format string, and also
was using the wrong format string (lu instead of d) to print the `cmd`
argument. It is corrected and the compiler warning resolved.

Signed-off-by: Matteo Golin <matteo.golin@gmail.com>

drivers/sensors/lis2mdl_uorb.c

@@ -1140,7 +1140,7 @@ static int lis2mdl_control(FAR struct sensor_lowerhalf_s *lower,
 
     default:
       err = -EINVAL;
-      snerr("Unknown command for LIS2MDL: lu\n", cmd);
+      snerr("Unknown command for LIS2MDL: %d\n", cmd);
       break;
     }