Improve transmitter disable.

Improve transmitter disable.

In some cases, the backend may stop processing Tx ring
requests in a timely manner. If this happens at the same time as ring
disable, then the Tx code could spin forever at dispatch IRQL.

This patch introduces a hard limit on how long the code will spin,
allowing a device disable or power transition to complete, albeit
at the cost of Tx requests being dropped or the ring being in
an indeterminate state. This has normally been seen at guest shutdown
where final ring state is of little consequence.

Signed-off-by: Martin Harvey <martin.harvey@citrix.com>