mld_set_config() treated any call with empty prev_mld as a fresh
configuration and triggered a full Reload all interfaces, even when the
new config was also empty (the typical path on non-MLD devices).
Reloading every BSS on each netifd reconf disrupted associated stations
including PMF-protected backhaul STAs, which would self-deauth after the
SA Query timeout.
Only treat the call as a new configuration when the new config is
actually non-empty.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
94fc66cce9