mmc: jz4740: fix return value check in jz4740_mmc_probe()

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(). Also there is already a error message within
devm_ioremap_resource(), so remove the dev_err call to avoid redundant
error message.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Chris Ball <cjb@laptop.org>