edac: only create sdram_scrub_rate where supported

Currently, sdram_scrub_rate sysfs node is created even if the device
doesn't support get/set the scub rate. Change the logic to only
create this device node when the operation is supported.

Reported-by: Felipe Balbi <balbi@ti.com>
Acked-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>