CHROMIUM: sd: Fix warning

CHROMIUM: sd: Fix warning

drivers/scsi/sd.c:2676:2: warning: assignment from incompatible pointer type [enabled by default]
drivers/scsi/sd.c:2913:2: warning: assignment from incompatible pointer type [enabled by default]

Since there's no way to return an error, do a WARN_ON() in the error
case instead.

BUG=chromium-os:34287
TEST=none (we need an automated test for error injection here)

Change-Id: Idaf0c0184e81ef7bac63b1deeda1a7690aa48a42
Signed-off-by: Olof Johansson <olofj@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/32830
Reviewed-by: Sonny Rao <sonnyrao@chromium.org>