hwmon: (pmbus/max20730) delete some dead code

The debugfs_create_dir() function never returns NULL.  Normal users are
not supposed to check the return value so the correct fix is just to
delete this check.

In the case where the debugfs_create_dir() fails, the function returns
NULL.  The other debugfs function check for NULL directory and handle
it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20201022070659.GA2817762@mwanda
Signed-off-by: Guenter Roeck <linux@roeck-us.net>