staging: mei: fix mei driver namespace

All global functions must start with mei_ to reduce the risk of name colisions

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Oren Weil <oren.jer.weil@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>