mei: store kind as enum

mei: store kind as enum

Simplify flows and prepare for future flexibility by
storing kind as enum and converting to string only in sysfs.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Co-developed-by: Reuven Abliyev <reuvenab@gmail.com>
Signed-off-by: Reuven Abliyev <reuvenab@gmail.com>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Link: https://patch.msgid.link/20260409125524.111530-2-alexander.usyskin@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>