hv_netvsc: remove locking in netvsc_send()

Packet scheduler guarantees there won't be multiple senders for the same
queue and as we use q_idx for multi_send_data the spinlock is redundant.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>