wifi: iwlwifi: mvm: d3: fix IPN byte order

The IPN is reported by the firmware in 6 bytes little endian,
but mac80211 expects big endian so it can do memcmp() on it.
We used to store this as a u64 which was filled in the right
way, but never used. When implementing that it's used, we
changed it to just be 6 bytes, but lost the conversion. Add
it back.

Fixes: 04f78e24

 ("wifi: iwlwifi: mvm: Add support for IGTK in D3 resume flow")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240129211905.138ed8a698e3.I1b66c386e45b5392696424ec636474bff86fd5ef@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>