mac80211_hwsim: allow setting iftype support

The mac80211_hwsim driver hard codes its supported interface types. For
testing purposes it would be valuable to allow changing these supported
types in order to simulate actual drivers than support a limited set of
iftypes. A new attribute was added to allow this:

- HWSIM_ATTR_IFTYPE_SUPPORT
	A u32 bit field of supported NL80211_IFTYPE_* bits

This will only enable/disable iftypes that mac80211_hwsim already
supports.

In order to accomplish this, the ieee80211_iface_limit structure needed
to be built dynamically to only include limit rules for iftypes that
the user requested to enable.

Signed-off-by: James Prestwood <james.prestwood@linux.intel.com>
[fix some indentation, add netlink error string]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>