iwlwifi: revamp tx scheduler byte count tables handling

This moves byte count tables to tx domain removing completely
ambivalent shared data. Changes handling of allocation
byte count tables and keep warm consistent memory
Moves general tx scheduler definitions from iwl-4956-hw.h
to iwl-fh.h

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>