drivers: re-use calibration topic

E.g. instead of defining a gyro_scale struct in drv_gyro.h, use the
gyro_calibration message.

src/drivers/hmc5883/hmc5883.cpp

@@ -1075,23 +1075,21 @@ int HMC5883::calibrate(struct file *filp, unsigned enable)
 	// XXX do something smarter here
 	int fd = (int)enable;
 
-	struct mag_scale mscale_previous = {
-		0.0f,
-		1.0f,
-		0.0f,
-		1.0f,
-		0.0f,
-		1.0f,
-	};
+	struct mag_calibration_s mscale_previous;
+	mscale_previous.x_offset = 0.0f,
+	mscale_previous.x_scale = 1.0f,
+	mscale_previous.y_offset = 0.0f,
+	mscale_previous.y_scale = 1.0f,
+	mscale_previous.z_offset = 0.0f,
+	mscale_previous.z_scale = 1.0f,
 
-	struct mag_scale mscale_null = {
-		0.0f,
-		1.0f,
-		0.0f,
-		1.0f,
-		0.0f,
-		1.0f,
-	};
+	struct mag_calibration_s mscale_null;
+	mscale_null.x_offset = 0.0f,
+	mscale_null.x_scale = 1.0f,
+	mscale_null.y_offset = 0.0f,
+	mscale_null.y_scale = 1.0f,
+	mscale_null.z_offset = 0.0f,
+	mscale_null.z_scale = 1.0f,
 
 	float sum_excited[3] = {0.0f, 0.0f, 0.0f};