iwlwifi: fix bug in tx byte count table

When setting invalid byte count in txq byte count table, read pointer
should be used instead of write pointer.

Reported-by: Guo, Chaohong <chaohong.guo@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>