iwlwifi: disable 8K A-MSDU by default

iwlwifi: disable 8K A-MSDU by default

Supporting 8K A-MSDU means that we need to allocate order 1
pages for every Rx packet. Even when there is no traffic.
This adds stress on the memory manager. The handling of
compound pages is also less trivial for the memory manager
and not using them will make the allocation code run faster
although I didn't really measure.
Eric also pointed out that having huge buffers with little
data in them is not very nice towards the TCP stack since
the truesize of the skb is huge. This doesn't allow TCP
to have a big Rx window.
See https://patchwork.kernel.org/patch/2167711/ for details.

Note that very few vendors will actually send A-MSDU.
Disable this feature by default.

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