iwlwifi: mvm: add IBSS support

At the firmware level, IBSS support has similar programming
requirements as AP/GO support, so use the same functions with
just small differences.

With IBSS only a single virtual interface can be used, so no
changes in the advertised interface combinations are needed.

For now, don't use hardware crypto for the GTKs in IBSS mode,
the firmware should support it though.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>