debugfs_create_*() returns NULL if an error occurs, returns -ENODEV
when debugfs is not enabled in the kernel.

Comparing to PATCH v1, because clk_debugfs_init is included in
"#if defined CONFIG_DEBUG_FS", we only need to check NULL return.
Thanks Li Zefan <lizf@cn.fujitsu.com>

debugfs_create_u8() and other function's return value's checking method are
also fixed in this patch.
Signed-off-by: NZhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>