i40e: Fix client interaction

This patch fixes a problem in the client interface that
was causing random stack traces in RDMA driver load and
unload tests.  This patch fixes the problem by checking
for an existing client before trying to open it.  Without
this patch, there is a timing related null pointer deref.

Change-ID: Ib73d30671a27f6f9770dd53b3e5292b88d6b62da
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>