iwlwifi: fix a bug in iwl_ssid_exist()

Rather than always comparing a given SSID to the first SSID of the
IE list, compare it to the the iterator one. That way duplicates can
be found.

Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)

(cherry picked from commit ee7077f24f5b02bde8cf5c202848128f18733398)
(cherry picked from commit a513f65b92b672830c89acfa04b93c3b77d3f97d)