i7core_edac: Introduce free_i7core_dev

Have a method to make a couple with alloc_i7core_dev() previously
introduced.  Using in pair will help proper resource handling.

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