thermal: exynos: Use IS_ERR() because regulator cannot be NULL

The NULL check in probe's error path is not needed because in that time
the regulator cannot be NULL (regulator_get() returns valid pointer or
ERR_PTR).

Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>