iio: adc: rzg2l_adc: Remove unnecessary channel check from rzg2l_adc_read_label()

Remove unnecessary channel check from rzg2l_adc_read_label(), as the
channel error handling is already done in probe(). Therefore no need to
validate at runtime.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20220515060337.16513-3-biju.das.jz@bp.renesas.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>