wifi: iwlwifi: mvm: fix initialization of a return value

In iwl_mvm_mld_update_sta(), if the flow doesn't enter
for_each_sta_active_link(), the default value is returned.
Set this default to -EINVAL instead of 0 to better reflect
this.

Signed-off-by: Mukesh Sisodiya <mukesh.sisodiya@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230514120631.98b7e3aacf0b.I2fc274dd7e374ef7fac8e26d71c9cd73323da665@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>