e1000e: Reset packet state after emptying Tx queue

e1000e: Reset packet state after emptying Tx queue

Keeping Tx packet state after the transmit queue is emptied has some
problems:
- The datasheet says the descriptors can be reused after the transmit
  queue is emptied, but the Tx packet state may keep references to them.
- The Tx packet state cannot be migrated so it can be reset anytime the
  migration happens.

Always reset Tx packet state always after the queue is emptied.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>