iwlwifi: pcie: gen2: use DMA pool for byte-count tables

Since the recent patch in this area, we no longer allocate 64k
for a single queue, but only 1k, which still means a full page.
Use a DMA pool to reduce this further, since we will have a lot
of queues in a typical system that can share pages.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200425130140.6e84c79aea30.Ie9a417132812d110ec1cc87852f101477c01cfcb@changeid