wifi: iwlwifi: mvm: fix beacon template/fixed rate

wifi: iwlwifi: mvm: fix beacon template/fixed rate

During the development of the rate changes, I evidently made
some changes that shouldn't have been there; beacon templates
with rate_n_flags are only in old versions, so no changes to
them should have been necessary, and evidently broke on some
devices. This also would have broken fixed (injection) rates,
it would seem. Restore the old handling of this.

Fixes: dabc88cb3b78 ("wifi: iwlwifi: handle v3 rates")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220558
Reviewed-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20251008112044.3bb8ea849d8d.I90f4d2b2c1f62eaedaf304a61d2ab9e50c491c2d@changeid
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>