thermal: exynos: Reduce severity of too early temperature read

Thermal core tries to read temperature during sensor registering in
thermal_zone_of_sensor_register().  In that time Exynos TMU driver and
hardware are not yet initialized.  Commit 0eb875d8 ("thermal:
exynos: Reading temperature makes sense only when TMU is turned on")
added a boolean flag to prevent reading bogus temperature in such
case but it exposed warning message during boot:
	[    3.864913] thermal thermal_zone0: failed to read out thermal zone (-22)

Return EAGAIN in such case to skip omitting such message because it
might mislead user.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

drivers/thermal/samsung/exynos_tmu.c

@@ -666,8 +666,14 @@ static int exynos_get_temp(void *p, int *temp)
 	struct exynos_tmu_data *data = p;
 	int value, ret = 0;
 
-	if (!data || !data->tmu_read || !data->enabled)
+	if (!data || !data->tmu_read)
 		return -EINVAL;
+	else if (!data->enabled)
+		/*
+		 * Called too early, probably
+		 * from thermal_zone_of_sensor_register().
+		 */
+		return -EAGAIN;
 
 	mutex_lock(&data->lock);
 	clk_enable(data->clk);