hv_netvsc: Wait for completion on request SWITCH_DATA_PATH

The completion indicates if NVSP_MSG4_TYPE_SWITCH_DATA_PATH has been
processed by the VSP. The traffic is steered to VF or synthetic after we
receive this completion.

Signed-off-by: Long Li <longli@microsoft.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>