i7core_edac: Introduce i7core_unregister_mci

In i7core_probe, when setup of mci for 2nd or later socket failed,
we should cleanup prepared mci for 1st socket or so before "put" of
all devices.

So let have i7core_unregister_mci that can be shared between here
and i7core_remove.

While here fix a typo "hanler".

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>