wil6210: add HW write-back option in TX descriptor

Map BIT 9 in TX DMA DWARD 0 as HW write back option.
We must turn on this option in the last TX descriptor,
this is required for old HW compatability.
This option indicate to HW that WB is required for this descriptor.

Signed-off-by: Kirshenbaum Erez <erezk@wilocity.com>
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>