hwmon: (nct6775) Fix potential Spectre v1

hwmon: (nct6775) Fix potential Spectre v1

val can be indirectly controlled by user-space, hence leading to
a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

vers/hwmon/nct6775.c:2698 store_pwm_weight_temp_sel() warn: potential
spectre issue 'data->temp_src' [r]

Fix this by sanitizing val before using it to index data->temp_src

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
CVE-2017-5753

(cherry picked from commit d49dbfade96d5b0863ca8a90122a805edd5ef50a)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>