hwmon: (ad7414) fix checkpatch issues

fixed:
WARNING: simple_strtol is obsolete, use kstrtol instead
#133: FILE: ad7414.c:133:
+	long temp = simple_strtol(buf, NULL, 10);

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>