wil6210: find TX vring only if vring_tx_data is enabled

In TX vring allocation, vring_tx->va is allocated before WMI command to
configure the vring is sent to the device. As the WMI command can take
time to complete, it can lead to scenarios where vring_tx->va is not NULL
but vring is still not enabled.

This patch adds a check that vring_tx_data is enabled before returning
a valid TX vring.

Change-Id: I21bad8fdbb7bb0ff8fe0447efb94dbfd69f4e6b0
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Git-commit: b729aaf066b013458e95cd6c69a5e371eb9ce355
Git-repo: https://github.com/kvalo/ath.git


CRs-Fixed: 971604
Signed-off-by: Maya Erez <merez@codeaurora.org>