mei: introduce mei_data2slots wrapper

Introduce mei_data2slots wrapper for sake of
readability. This wrapper close up the open code
for computing slots from a message length:
rond up dwords count from payload and header byte size

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