iwlwifi: allow using multiple contexts

We're now ready to start using multiple contexts.
We do this by keeping track of the valid interface
types per context (exclusive [ibss] and normal)
and checking which context is "free" when a new
interface is added.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>