Set sf0x min dist to 30 cm, 0 cm is outside of spec for sf02.

src/drivers/sf0x/sf0x.cpp

@@ -89,7 +89,7 @@ static const int ERROR = -1;
 
 #define SF0X_CONVERSION_INTERVAL	83334
 #define SF0X_TAKE_RANGE_REG		'd'
-#define SF02F_MIN_DISTANCE		0.0f
+#define SF02F_MIN_DISTANCE		0.3f
 #define SF02F_MAX_DISTANCE		40.0f
 
 // designated SERIAL4/5 on Pixhawk