wil6210: prevent usage of incorrect TX hwtail

txdata->enabled is used in order to determine if the TX vring
is valid. As the data transmit is handled in a different context,
in case txdata->enabled is set before vring->hwtail is updated,
an old or corrupted vring->hwtail can be used.
Protect setting of txdata->enabled and vring->hwtail to prevent a
case where TX vring start handling TX packets before setting
vring->hwtail.

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