rockchip: adc: rockchip-saradc: factor out stop callback

SARADC v2 doesn't have a stop mechanism once in single mode. In series
conversion, the logic is different anyway. Therefore, let's abstract
this function so that it can be provided from the udevice.data pointer.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>