cfg80211: fix NAN bands definition

The nl80211_nan_dual_band_conf enumeration doesn't make much sense.
The default value is assigned to a bit, which makes it weird if the
default bit and other bits are set at the same time.

To improve this, get rid of NL80211_NAN_BAND_DEFAULT and add a wiphy
configuration to let the drivers define which bands are supported.
This is exposed to the userspace, which then can make a decision on
which band(s) to use.  Additionally, rename all "dual_band" elements
to "bands", to make things clearer.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>