hwmon: (pmbus) Increase attribute name size

Some hwmon sysfs attributes have a length of 20 bytes (plus terminating 0).
I2C_NAME_SIZE is defined as 20 and thus can not be used to define the length
of hwmon sysfs attributes. Replace it with PMBUS_NAME_SIZE, set to 24.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>