iio:temperature:mlx90632: Convert polling while loop to regmap

Reduce number of lines and improve readability to convert polling while
loops to regmap_read_poll_timeout.

Signed-off-by: Crt Mori <cmo@melexis.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200818213737.140613-4-cmo@melexis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>