iwlwifi: mvm: honor the max_amsdu_subframes limit

A peer can limit the number of subframes it can handle in a
single A-MSDU.  Honor this limit.

Note that the smallest limit is 8, and we are very unlikely to reach
that limit. So this isn't really a big deal.

Fixes: a6d5e32f ("iwlwifi: mvm: send large SKBs to the transport")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>