Getting the BSS station vif is something that may be needed by other
parts of the code.  So, instead of having an iterator specifically for
d3, change it into a generic one in utils.c.  Additionally, add a
iwl_mvm_get_bss_vif() function to make it easier to retrieving it.
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>