firmware: exynos-acpm: Use unsigned int for acpm_pmic_linux_errmap index

firmware: exynos-acpm: Use unsigned int for acpm_pmic_linux_errmap index

acpm_pmic_to_linux_err() uses an unsigned integer obtained from messages
as index of array to map them to error codes.  Array index cannot be
negative, so make that explicit.

Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260219-firmare-acpm-counted-v2-1-e1f7389237d3@oss.qualcomm.com
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>