mac80211: ensure no limits on station rhashtable

By default, the rhashtable logic will fail to insert
objects if the key-chains are too long and un-balanced.

In the degenerate case where mac80211 is creating many
virtual interfaces connected to the same peer(s), this
case can happen.

St insecure_elasticity to true to allow chains to grow
as long as needed.

Signed-off-by: Ben Greear <greearb@candelatech.com>
[remove message, change commit message slightly]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>