i40e: Fix a bug where a client close can be called before an open is complete

The client->open call in this path was not protected with the
client instance mutex, and hence the client->close can get initiated
before the open completes.

Change-Id: I0ed60c38868dd3f44966b6ed49a063d0e5b7edf5
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>