Staging: mei: move the mei code out of staging

It's been cleaned up, and there's nothing else left to do, so move it
out of staging into drivers/misc/ where all can use it now.

Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: Oren Weil <oren.jer.weil@intel.com>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>