iwlwifi: move virtual interface pointer into context

iwlwifi occasionally needs to find the virtual
interface pointer to give it to mac80211, but right
now it only keeps one. Move it into the context so
that we can keep one pointer each.

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