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

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: 7ef01f228c9f ("thermal/debugfs: Add thermal debugfs information for mitigation episodes")
Fixes: 755113d76786 ("thermal/debugfs: Add thermal cooling device debugfs information")
Signed-off-by: Yang Ruibin <[email protected]>
Link: https://patch.msgid.link/[email protected]
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <[email protected]>