wil6210: handle tx completions only if vring tx data is enabled

wil_vring_fini_tx is called in disconnect in order to free the
allocated vrings.
wil_vring_fini_tx is disabling the vring_tx_data before napi_synchronize
is called in order to avoid the tx handling of this vring, while
wil_vring_free is called only after napi finished the current handling
of the tx completed packets.
Due to that, in case of disconnect, wil6210_netdev_poll_tx can be called
when vring->va is not NULL but vring_tx_data[i]->enabled is already
disabled.

This patch checks vring_tx_data[i]->enabled in wil6210_netdev_poll_tx
to prevent handling of disabled vrings.

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>