EDAC/igen6: Skip absent memory controllers

Some BIOS versions may fuse off certain memory controllers and set the
registers of these absent memory controllers to ~0. The current igen6_edac
mistakenly enumerates these absent memory controllers and registers them
with the EDAC core.

Skip the absent memory controllers to avoid mistakenly enumerating them.

Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20250408132455.489046-2-qiuxu.zhuo@intel.com