iwlwifi: mvm: add WEP RX hardware offload support

iwlwifi: mvm: add WEP RX hardware offload support

In the original driver, we decided to not implement WEP RX hardware
offload because of a quirk with the firmware API - it allows setting
global WEP keys that then get used for all virtual interfaces, which
is clearly wrong if more than one exists, and it allows setting per-
station keys but then separates multicast and unicast keys.

In order to implement WEP RX hardware offload, work around these
limitations by uploading each WEP key twice, once as multicast and
once as unicast, but point them both to the same key slot (offset)
and use the same key material so the slot overwrite on the second
upload doesn't actually change anything. Upon removal, also remove
the key twice so the station no longer references it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>