x86, microcode: Correct sysdev_add error path

When we encounter an error while initting the microcode driver on a CPU,
we must undo the previously added sysfs group.

Cc: Tigran Aivazian <tigran@aivazian.fsnet.co.uk>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Acked-by: Andreas Herrmann <Andreas.Herrmann3@amd.com>