wifi: nl80211: enable setting the link address at new station

Since for an MLD station the default link is added together
with the add station command, allow also setting the link
MAC address.
Otherwise, it is needed to use the modify link API only
for setting the link MAC address.

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>