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>