iwlwifi: remove iwl4965_tx_cmd

This patch removes iwl4965_tx_cmd function and splits its content to
iwl4965_hw_build_tx_cmd_rate, iwl4965_build_tx_cmd_basic,
and iwl4965_tl_get_stats function. The latest one will be deprecated
when traffic load will move to rate scale module.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>