regulator: core: Fix more error checking for debugfs_create_dir()

In case of failure, debugfs_create_dir() does not return NULL, but an
error pointer.  Most incorrect error checks were fixed, but the one in
create_regulator() was forgotten.

Fix the remaining error check.

Fixes: 2bf1c45b

 ("regulator: Fix error checking for debugfs_create_dir")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/ee980a108b5854dd8ce3630f8f673e784e057d17.1685013051.git.geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>