iwlwifi: contextify-stations-completely

The microcode tracks stations per context, so
the driver needs to do that as well for adding,
deleting and restoring them, especially in the
implicit removal case when we send an RXON.

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