hwmon: (pmbus/dps920ab) Delete some dead code

The debugfs_create_dir() function returns error pointers, it doesn't
return NULL.  But debugfs functions don't need to be checked in normal
situations and we can just delete this code.

Fixes: 1f442e213ce5 ("hwmon: (pmbus) Add driver for Delta DPS-920AB PSU")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YMyjmR54ErLtc1sH@mwanda


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