EDAC: Make dev_attr_sdram_scrub_rate static

The dev_attr_sdram_scrub_rate is not declared in a header or used
anywhere else, so make it static to fix the following warning:

  drivers/edac/edac_mc_sysfs.c:816:1: warning: symbol
  'dev_attr_sdram_scrub_rate' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/1465407356-7357-1-git-send-email-ben.dooks@codethink.co.uk


Signed-off-by: Borislav Petkov <bp@suse.de>