iwlagn: rewrite HW crypto

As I just discovered while doing WoWLAN, HW crypto
is done wrong for GTKs: they should be programmed
for the AP station ID (in the managed mode case)
and the HW can actually deal with multiple group
keys per station as well (which is useful in IBSS
RSN but that I've chosen not to use this).

To fix all this, modify the way keys are sent to
the device and key offsets are allocated. After
these changes, key offsets are stored into the
hw_key_idx which we can then track for the key
lifetime, not relying on our sta_cmd array. WEP
default keys get special treatment, of course.

Additionally, since I had the API for it, we can
now pre-fill TKIP phase 1 keys for RX now that we
can obtain the P1K from mac80211, a capability I
had added for WoWLAN initially.

Finally, some keys simply don't need to be added
into the device's key cache -- a key that won't
be used for RX is only needed in the TX header,
so "pretend" to have accepted any key without
adding it into the device -- no need to use up
key space there for it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>