iio: adc: meson_saradc: Use regmap_read_poll_timeout() for busy wait

Simplify busy wait stages by using regmap_read_poll_timeout().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com> # GXM VIM2
Link: https://lore.kernel.org/r/20220603100004.70336-6-andriy.shevchenko@linux.intel.com


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