spi: hisi-kunpeng: Fix error checking

The function debugfs_create_dir returns ERR_PTR if an error occurs,
and the appropriate way to verify for errors is to use the inline
function IS_ERR. The patch will substitute the null-comparison with
IS_ERR.

Suggested-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Yeqi Fu <asuk4.q@gmail.com>
Link: https://lore.kernel.org/r/20230518052515.368978-1-asuk4.q@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>