iwlwifi: mvm: change the iwl_mvm_d3_iface_iterator into a generic function

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>