i40e: Correcting mutex usage in client code

Correcting the mutex usage, in client_subtask(), mutex_unlock has
to be called just before client_del_instance() since this function opens
and later closes the same mutex again.
Similarly in client_is_registered removing the mutex since it closes
the mutex twice.

This is a patch suggested by RDMA team.

Change-ID: Icce519c266e4221b8a2a72a15ba5bf01750e5852
Signed-off-by: Avinash Dayanand <avinash.dayanand@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>