mei: drop unneeded client NULL check in cb structure

The pointer to client in the callback structure (cb->cl)
can't be NULL with current locking.
We can drop check and warnings as in some cases this just
uselessly complicates the code flow.

Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>