hv_netvsc: fix race that may miss tx queue wakeup

When the ring buffer is almost full due to RX completion messages, a
TX packet may reach the "low watermark" and cause the queue stopped.
If the TX completion arrives earlier than queue stopping, the wakeup
may be missed.

This patch moves the check for the last pending packet to cover both
EAGAIN and success cases, so the queue will be reliably waked up when
necessary.

Reported-and-tested-by: Stephan Klein <stephan.klein@wegfinder.at>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>