iwlwifi: mvm: allocate reorder buffer according to need

Now that we may have up to 256 entries per reorder buffer, and possibly up
to 16 queues, we can use a LOT of memory for this (64k for each station).
Allocate it according to what we need, which is of course much less for HT
stations (only 16k at a max of 16 queues).

However, this comes at the expense of complicating the code a bit to
calculate the right entry structure to use for each frame.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>