EDAC/mc: Fix edac_mc_find() in case no device is found

The function should return NULL in case no device is found, but it
always returns the last checked mc device from the list even if the
index did not match. Fix that.

I did some analysis why this did not raise any issues for about 3 years
and the reason is that edac_mc_find() is mostly used to search for
existing devices. Thus, the bug is not triggered.

 [ bp: Drop the if (mci->mc_idx > idx) test in favor of readability. ]

Fixes: c73e8833 ("EDAC, mc: Fix locking around mc_devices list")
Signed-off-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lkml.kernel.org/r/20190514104838.15065-1-rrichter@marvell.com