mei: rename enum mei_cb_major_types to enum mei_cb_file_ops

1. Rename mei_cb_major_types to more understandable mei_cb_file_ops
2. Rename member struct mei_cl_cb of this type to simple 'fop_type'
3. Add kernel doc for the type

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