i40e: Move the mutex lock in i40e_client_unregister

We need to lock the client list around the i40e_client_release call to
prevent the release from interrupting the client instances while they are
being added.

Change-Id: I99993f20179aaf8730207833e7d0869d2ccffa1d
Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>