EDAC, pnd2: Return proper error value from apl_rd_reg()

Add code comment to make it clear that the fall-through is intentional
and, OR ret with its previous value to avoid overwriting it so that
callers can check the correct return value.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Cc: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/20170622220535.GA4896@embeddedgus


[ Massage a bit. ]
Signed-off-by: Borislav Petkov <bp@suse.de>