cfg80211: Add support for 60GHz band channels 5 and 6

The current support in the 60GHz band is for channels 1-4.
Add support for channels 5 and 6.
This requires enlarging ieee80211_channel.center_freq from u16 to u32.

Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>