mailbox: mailbox-test: Fix debugfs_create_dir error checking

mailbox: mailbox-test: Fix debugfs_create_dir error checking

[ Upstream commit 3acf1028f5003731977f750a7070f3321a9cb740 ]

The debugfs_create_dir() function returns ERR_PTR() on error, not NULL.
The current null-check fails to catch errors.

Use IS_ERR() to correctly check for errors.

Fixes: 8ea4484d0c2b ("mailbox: Add generic mechanism for testing Mailbox Controllers")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>