wil6210: avoid Tx descriptor double write

Non-cacheable Tx descriptor for the last fragment of
multi-fragment frame used to be written back twice.

Refactor code to always write non-cacheable descriptor
only once

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