ARM: OMAP2+: Fix an IS_ERR() vs NULL check in _get_pwrdm()

The of_clk_get() function returns error pointers, it never returns NULL.

Fixes: 4ea3711a ("ARM: OMAP2+: omap-iommu.c conversion to ti-sysc")
Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>