Reset the Tx timer when iwx(4) firmware sends a BA notification.

Reset the Tx timer when iwx(4) firmware sends a BA notification.

When multiple frames are sent in a batch on a Tx aggregation queue our
current firmware version does not provide the IWX_TX_CMD notification.
Older versions used to provide this (as observed on iwm(4) devices),
but our current firmware only sends IWX_BA_NOTIF.

This means we need to reset the Tx timer upon BA_NOTIF in order to
avoid a bogus "device timeout" trigger from our watchdog handler.
Do this as soon as the BA notification has been validated.