mmc: sdhci-esdhc-imx: fix return value check in sdhci_esdhc_imx_probe()

In case of error, the function devm_kzalloc() returns NULL pointer not
ERR_PTR(). The IS_ERR() test in the return value check should be
replaced with NULL test.

Fixes: fadac7488064 ("mmc: sdhci-esdhc-imx: add CMDQ support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>