scsi: ufs: ufs-exynos: Fix return value check in exynos_ufs_init()

scsi: ufs: ufs-exynos: Fix return value check in exynos_ufs_init()

In case of error, the function devm_ioremap_resource() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR().

Link: https://lore.kernel.org/r/20200618133837.127274-1-weiyongjun1@huawei.com
Fixes: 55f4b1f73631 ("scsi: ufs: ufs-exynos: Add UFS host support for Exynos SoCs")
Reported-by: Hulk Robot <hulkci@huawei.com>
Acked-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>