iwlwifi: implement switching iftype while up

Implement switching the interface while an
interface is up in iwlwifi. Interfaces have
to stay on the context they were created on.

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