hwmon: (pmbus) Export pmbus device debugfs directory entry

Pmbus client drivers, if they want to use debugfs, should use the same
root directory as the pmbus debugfs entries are using. Therefore, export
the device dentry for the pmbus client.

Signed-off-by: Edward A. James <eajames@us.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>