thermal/debugfs: Fix the NULL vs IS_ERR() confusion in debugfs_create_dir()

The debugfs_create_dir() return value is never NULL, it is either a
valid pointer or an error one.

Use IS_ERR() to check it.

Fixes: 7ef01f22 ("thermal/debugfs: Add thermal debugfs information for mitigation episodes")
Fixes: 755113d7 ("thermal/debugfs: Add thermal cooling device debugfs information")
Signed-off-by: Yang Ruibin <11162571@vivo.com>
Link: https://patch.msgid.link/20240821075934.12145-1-11162571@vivo.com


[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>