scsi-disk: add 'fall through' comment to switch VERIFY cases

Commit 166dbda7e131 added some extra cases to a switch() such
that the existing code is intended to fall through the new
case statements. It's clear from the commit that this is
intentional, but less clear to subsequent readers of the
code, and not clear at all to static analysis tools like
Coverity. Add a /* fall through */ comment to indicate the
intent. (Fixes CID 1368287.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
