staging: hv: fix counting of available buffer slots when send fails

Because the number of available buffer slots doesn't decrease for failed
sends, we should not call netvsc_xmit_completion(), which increase the
count of available slots. In this failed case, just free the memory is
enough.

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>