EDAC: Don't allow empty DIMM labels

Updating dimm_label to an empty string does not make much sense. Change
the sysfs dimm_label store operation to fail a request when an input
string is empty.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: elliott@hpe.com
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/1443124767.25474.172.camel@hpe.com
Signed-off-by: Borislav Petkov <bp@suse.de>