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.

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>