CVE-2026-63936: iio: adc: mt6359: fix unchecked return value in mt6358_read_imp

In the Linux kernel, the following vulnerability has been resolved:

iio: adc: mt6359: fix unchecked return value in mt6358readimp

In mt6358readimp(), the variable valv is passed to regmapread() but the return value is not checked. If the read fails, valv remains uninitialized and its random stack content is subsequently reported as a measurement result.

Initialize valv to zero to ensure a predictable value is reported in case of bus failure and to prevent potential stack data leakage. This also satisfies static analyzers that might otherwise flag the variable as used uninitialized.