mei: cleanup slots to data conversions

Cleanup conversions between slots and data.
Define MEI_SLOT_SIZE instead of using 4 or sizeof(u32) across
the source code.

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