EDAC, i82975x: Add ioremap_nocache() error handling

EDAC, i82975x: Add ioremap_nocache() error handling

If ioremap_nocache() fails, it will return NULL. Which will then cause a
NULL-pointer dereference. Handle the returned value properly.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: "Arvind R." <arvino55@gmail.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/1484549092-11349-1-git-send-email-arvind.yadav.cs@gmail.com
[ Boris: massage commit message and improve error message. ]
Signed-off-by: Borislav Petkov <bp@suse.de>