spi: sprd-adi: checking for NULL instead of IS_ERR()

devm_ioremap_resource() returns error pointers, it never returns NULL.

Fixes: 7e2903cb ("spi: Add ADI driver for Spreadtrum platform")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>