Command NL80211_CMD_GET_WIPHY without any flag only returns channels
in the 2 GHz and 5 GHz bands, for backwards compatibility with old
userspace tools. To get the full list we need to pass attribute
NL80211_ATTR_SPLIT_WIPHY_DUMP (added in Linux 3.9 released in 2013),
and allow the handler to be called multiple times.

#1500