i7core_edac: Reduce args of i7core_get_onedevice

Since we need to pass the index of the entry, pass the table itself
instead of passing individual members of the table.

While here make it static.

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