wil6210: remove old Tx work-around

In the Tx, work around used to force destination index 0
to be used. This is no more necessary, as firmware supports
multiple destinations

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>