wifi: iwlwifi: mvm: define link_sta in the relevant scope

The link_sta pointer is now only used inside this if, so define it inside
its scope.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240510170500.e0a1dd285d9a.Id5ff16fb98af0c5f533aed6b5ec3cc856401e488@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>