Fix Coverity CID 1435957:  Memory - illegal accesses (OVERRUN):

>>> Overrunning array "suffixes" of 7 8-byte elements at element
    index 7 (byte offset 63) using index "idx" (which evaluates to 7).

Note, the biggest input value freq_to_str() can accept is UINT64_MAX,
which is ~18.446 EHz, less than 1000 EHz.

Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Message-id: 20201101215755.2021421-1-f4bug@amsat.org
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>