staging/mei: cleanup driver naming strings

1. use only mei_driver_name and remove define MEI_DRIVER_NAME
2. drop MEI_DEV_NAME and assign device name directly
3. drop mei_driver_string, it is not used

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