regulator: Fix error checking for debugfs_create_dir

This patch fixes the error checking in core.c in debugfs_create_dir.
The correct way to check if an error occurred is 'IS_ERR' inline function.

Signed-off-by: Osama Muhammad <osmtendev@gmail.com>
Suggested-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230515172938.13338-1-osmtendev@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>