iwlagn: prefer BSS context

If an interface type changes from a type that is
only supported on the PAN context (e.g. P2P GO)
to a type that is supported on the BSS context,
and the BSS context is not in use, then we need
to use the BSS context instead of changing the
device type within the context. To achieve this,
refuse the type change, which causes a down/up
cycle that will allocate the BSS context for the
interface.

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