staging: mei: expose misc interface instead of char device

Misc device provides everything MEI needs for registration,
it doesn't required separate driver class.

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