iwlwifi: move iwl_sta_modify_enable_tid_tx to iwl-sta.c

This patch moves iwl_sta_modify_enable_tid_tx into iwl-sta.c.

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