hwmon: (pmbus/max20730) use scnprintf() instead of snprintf()

The snprintf() function returns the number of characters which would
have been printed if there were enough space, but the scnprintf()
returns the number of characters which were actually printed.  If the
buffer is not large enough, then using snprintf() would result in a
read overflow and an information leak.

Fixes: 8910c0bd ("hwmon: (pmbus/max20730) add device monitoring via debugfs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20201022070824.GC2817762@mwanda


Signed-off-by: Guenter Roeck <linux@roeck-us.net>