gve: Reset Rx ring state in the ring-stop funcs

This does not fix any existing bug. In anticipation of the ndo queue api
hooks that alloc/free/start/stop a single Rx queue, the already existing
per-queue stop functions are being made more robust. Specifically for
this use case: rx_queue_n.stop() + rx_queue_n.start()

Note that this is not the use case being used in devmem tcp (the first
place these new ndo hooks would be used). There the usecase is:
new_queue.alloc() + old_queue.stop() + new_queue.start() + old_queue.free()

Tested-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Praveen Kaligineedi <pkaligineedi@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Signed-off-by: Shailend Chand <shailend@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>