iio: proximity: rfd77402: Use kernel helper for result polling

iio: proximity: rfd77402: Use kernel helper for result polling

Replace the manually written polling loop with read_poll_timeout(),
the kernel's standard helper for waiting on hardware status.This
makes the code easier to read.

Move the polling logic into a dedicated helper function, as it will
be reused by future updates.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Shrikant Raskar <raskar.shree97@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>