wifi: iwlwifi: fix potential use after free in iwl_mld_remove_link()

wifi: iwlwifi: fix potential use after free in iwl_mld_remove_link()

This code frees "link" by calling kfree_rcu(link, rcu_head) and then it
dereferences "link" to get the "link->fw_id".  Save the "link->fw_id"
first to avoid a potential use after free.

Fixes: d1e879ec600f ("wifi: iwlwifi: add iwlmld sub-driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/aNKCcKlbSkkS4_gO@stanley.mountain
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>