wil6210: fix _desc access in __wil_tx_vring_tso

_desc is defined in __wil_tx_vring_tso() and may not be set in
case len is 0, verify _desc is set.

Change-Id: I5f9aab4da683d6a535443e4b834ec887b044378a
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Git-commit: 9c6465979276c58d5972a959dd35b4d9aff52749
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git


Signed-off-by: Maya Erez <merez@codeaurora.org>