ena: Store ticks of last Tx cleanup

Store timestamp of last cleanup in Tx ring structure. This does not
change anything during normal operation of the driver but could be
useful when the device fails for some reason.

Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.

(cherry picked from commit d8aba82b5ca75f1a5bff609af141844c4fc9de70)