thermal: exynos: Defer probe if vtmu is present but not registered

The driver doesn't check if the regulator_get_optional return value is
-EPROBE_DEFER so it will wrongly assume that the regulator couldn't be
found just because the regulator driver wasn't registered yet, i.e:

exynos-tmu 10060000.tmu: Regulator node (vtmu) not found

In this case the return value should be propagated to allow the driver
probe function to be deferred until the regulator driver is registered.

Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>