22560 devices RFH uses different structures, which act similar
to the legacy rxq management lists - free and used list.

The iwl_rx_transfer_desc struct is part of the free list,
and consists of pointers to the empty rb's the driver wants to
pass to the fw.

The iwl_rx_completion_desc struct is part of the used list,
and consists of pointers to the buffer the fw filled up
with new rx, both commands and data, for the host.
Signed-off-by: Golan Ben Ami <golan.ben.ami@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>