iio/adjd_s311: Fix potential memory leak in adjd_s311_update_scan_mode()

Do not leak memory by updating pointer with potentially NULL realloc return value.
There is no need to preserve data in the buffer,
so replace krealloc() by kfree()-kmalloc() pair.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>