iwlwifi: mvm: add an option to dereference vif by id

Currently whenever we get firmware notification with mac id,
we iterate over all the interfaces to find the ID. This is a
bit cumbersome. Instead, adding an array of RCU pointers, like
we have for station IDs. This is not expensive space wise
since we have only up to 4 active MACs, and not complicated
code wise, since we have a clear point to init and de-init it.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>