OMAP: clock: fix compile warning

if building kernels without OMAP2 support, we
will see a warning such as:

arch/arm/mach-omap2/io.c: In function 'omap2_init_common_infrastructure':
arch/arm/mach-omap2/io.c:389:3: warning: statement with no effect
arch/arm/mach-omap2/io.c:391:3: warning: statement with no effect
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>

arch/arm/mach-omap2/clock2xxx.h

@@ -23,13 +23,13 @@ void omap2xxx_clk_prepare_for_reboot(void);
 #ifdef CONFIG_SOC_OMAP2420
 int omap2420_clk_init(void);
 #else
-#define omap2420_clk_init()	0
+#define omap2420_clk_init()	do { } while(0)
 #endif
 
 #ifdef CONFIG_SOC_OMAP2430
 int omap2430_clk_init(void);
 #else
-#define omap2430_clk_init()	0
+#define omap2430_clk_init()	do { } while(0)
 #endif
 
 extern void __iomem *prcm_clksrc_ctrl, *cm_idlest_pll;