iwlwifi: add support for 12K Receive Buffers

802.11ac allows A-MSDU that can be up to 12KB long. Since
an entire A-MSDU needs to fit into one single Receive
Buffer (RB), add support for big RBs.
Since this adds lots of pressure to the memory manager and
significantly increase the true_size of the RX buffers,
don't enable this by default.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>