samples: mei: use /dev/mei0 instead of /dev/mei

The device was moved from misc device to character devices
to support multiple mei devices.

Cc: <stable@vger.kernel.org> #v4.9+
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>