iio: adc: nxp-sar-adc: use field_get() for EOC bit check

iio: adc: nxp-sar-adc: use field_get() for EOC bit check

Use field_get() here now that runtime-mask support exists, and drop
the obsolete TODO. Since NXP_SAR_ADC_EOC_CH(c) is BIT(c), the
resulting !-test is semantically identical.

No functional change.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Piyush Patle <piyushpatle228@gmail.com>
Acked-by: Daniel Lezcano <daniel.lezcano@oss.qualcomm.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>