OpticalFlow: make surface_quality uint8_t

libraries/AP_OpticalFlow/AP_OpticalFlow.h

@@ -35,7 +35,7 @@ public:
     // raw sensor change in x and y position (i.e. unrotated)
     int      raw_dx, raw_dy;
     // image quality (below 15 you really can't trust the x,y values returned)
-    int      surface_quality;
+    uint8_t      surface_quality;
     // total x,y position
     int      x,y;
     // rotated change in x and y position

libraries/AP_OpticalFlow/AP_OpticalFlow_ADNS3080.cpp

@@ -165,7 +165,7 @@ void
 AP_OpticalFlow_ADNS3080::update(uint32_t now)
 {
     uint8_t motion_reg;
-    surface_quality = (uint16_t)read_register(ADNS3080_SQUAL);
+    surface_quality = read_register(ADNS3080_SQUAL);
     hal.scheduler->delay_microseconds(50);
 
     // check for movement, update x,y values