soc: samsung: exynos-asv: handle reading revision register error

If regmap_read() fails, the product_id local variable will contain
random value from the stack.  Do not try to parse such value and fail
the ASV driver probe.

Fixes: 5ea42859 ("soc: samsung: Add Exynos Adaptive Supply Voltage driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Link: https://lore.kernel.org/r/20201207190517.262051-3-krzk@kernel.org