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 mt6358_read_imp

In mt6358_read_imp(), the variable val_v is passed to regmap_read()

but the return value is not checked. If the read fails, val_v remains

uninitialized and its random stack content is subsequently reported

as a measurement result.

Initialize val_v 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.