nl80211: support receiving split frequencies

In order to get more details about each frequency we will need to set
NL80211_ATTR_SPLIT_WIPHY_DUMP in the future. This will result in our
callback being called multiple times. Modify it to support such a
scenario:
1) Start putting new frequencies after the last set one
2) Make sure that attribute is set before iterating it

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Jo-Philipp Wich <jo@mein.io>