iio: frequency: ad9523: Increase sleep time in ad9523_store_eeprom

This patch increases sleep time in ad9523_store_eeprom to 20ms since
it isn't timing critical. It fixes the following checkpatch.pl warning:
WARNING: msleep < 20ms can sleep for up to 20ms

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>