gve: Fix timestamp invalidation for DQO queue formats

We need to invalidate timestamps when a TX queue is cleared so that the
TX timeout detection callout does not mistakenly fire for cleared
packets. When using DQO queue formats, timestamps are set on the pending
packet array whose length is not the same as the length of the
descriptor ring itself. This commit fixes logic which invalidated the
wrong number of pending packets.

Signed-off-by: Jasper Tran O'Leary <jtranoleary@google.com>

Fixes: 3d2957336c7d ("gve: Add callout to detect and handle TX timeouts")
Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D50688