OV7670 fixes (#484)

* ov7670: fix ov7670_frame_control()

Last value of array (VREF) was overwritten by end marker

* OV7670: Adapt QQVGA VSTART value to remove bar on top

Remove empty bar at the top of the frame. This is more visible on
OV7675, but also present on OV7670

sensors/ov7670.c

@@ -194,8 +194,8 @@ struct regval_list frame[7];
     frame[5].value = (((vstop & 0x02) << 2) | (vstart & 0x02));
 
     /* End mark */
-    frame[5].reg_num = 0xFF;
+    frame[6].reg_num = 0xFF;
-    frame[5].value = 0xFF;
+    frame[6].value = 0xFF;
 
     return ov7670_write_array(sensor, frame);
 }
@@ -277,7 +277,7 @@ static int set_framesize(sensor_t *sensor, framesize_t framesize)
 	    case FRAMESIZE_QQVGA:
             if( (ret = ov7670_write_array(sensor, ov7670_qqvga)) == 0 ) {
                 /* These values from Omnivision */
-                ret = ov7670_frame_control(sensor, 158, 14, 10, 490);
+                ret = ov7670_frame_control(sensor, 158, 14, 12, 490);
             }
         break;