ARM: imx: fix the return value check in imx_clk_busy_divider()

In case of error, the function clk_register() returns ERR_PTR()
no NULL pointer. The NULL test in the return value check should
be replaced with IS_ERR().

dpatch engine is used to auto generated 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>