mei: export mei.h for the user space

The header exports API for application layer

1. move under include/linux and add to  the export list
2. update include path n the sources
3. update TODO

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