mei: use type struct mei_cl *cl instead of void in struct mei_cb

We can use correct type 'struct mei_cl' instead of
'void *' for file_private in the struct mei_cb
as there is no other type assigned to this member of the structure

We rename the member from file_private to cl

Remove about 10 lines of declarations of temporary variables
used for type casting

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