ARM: OMAP1: clock: Use IS_ERR_OR_NULL() to clean code

Use IS_ERR_OR_NULL() to make the code cleaner.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>