staging: hv: fix the return status of netvsc_start_xmit()

Fix the return status, so the upper layer will retry if transmission
fails.

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>