iwlwifi: pcie: optimise struct iwl_rx_mem_buffer layout

On 64-bit machines, struct iwl_rx_mem_buffer has a lot of
padding due to the use of pointers after the small items.
Move the list entry before them, and while at it also add
documentation for it.

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.20210802170640.6a62255b3df0.I47bb36530a3c2cdbd73454c796ce608ee2a32a6c@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>