iio: adc: rockchip_saradc: fix bitmask for channels on SARADCv2

iio: adc: rockchip_saradc: fix bitmask for channels on SARADCv2

The SARADCv2 on RK3588 (the only SoC currently supported that has an
SARADCv2) selects the channel through the channel_sel bitfield which is
the 4 lowest bits, therefore the mask should be GENMASK(3, 0) and not
GENMASK(15, 0).

Fixes: 757953f8ec69 ("iio: adc: rockchip_saradc: Add support for RK3588")
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20240223-saradcv2-chan-mask-v1-1-84b06a0f623a@theobroma-systems.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>