staging: hv: fix counting of #outstanding-sends in failed sends

If the packet failed to be sent, we shouldn't count it as the
number of outstanding sends.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>