ARM: imx: fix return value check in imx3_init_l2x0()

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

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>