iio: adc: mt6359: fix unchecked return value in mt6358_read_imp

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.

Fixes: 3587914bf61d ("iio: adc: Add support for MediaTek MT6357/8/9 Auxiliary ADC")
Signed-off-by: Salah Triki <salah.triki@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>