i40e: remove a stray unlock

We shifted the locking around a bit but forgot to delete this unlock so
now it can unlock twice.

Fixes: cd3be169

 ('i40e: Move the mutex lock in i40e_client_unregister')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>