omap2plus: voltage: Trivial warning fix 'no return statement'

Fix below build warnings

 CC      arch/arm/mach-omap2/common.o
  CC      arch/arm/mach-omap2/gpio.o
In file included from arch/arm/plat-omap/include/plat/omap_hwmod.h:38,
                 from arch/arm/mach-omap2/gpio.c:25:
arch/arm/plat-omap/include/plat/voltage.h: In function 'omap_voltage_register_pmic':
arch/arm/plat-omap/include/plat/voltage.h:137: warning: no return statement in function returning non-void
  CC      arch/arm/mach-omap2/dma.o
In file included from arch/arm/plat-omap/include/plat/omap_hwmod.h:38,
                 from arch/arm/mach-omap2/dma.c:32:
arch/arm/plat-omap/include/plat/voltage.h: In function 'omap_voltage_register_pmic':
arch/arm/plat-omap/include/plat/voltage.h:137: warning: no return statement in function returning non-void
  CC      arch/arm/mach-omap2/wd_timer.o
In file included from arch/arm/plat-omap/include/plat/omap_hwmod.h:38,
                 from arch/arm/mach-omap2/wd_timer.c:15:
arch/arm/plat-omap/include/plat/voltage.h: In function 'omap_voltage_register_pmic':
arch/arm/plat-omap/include/plat/voltage.h:137: warning: no return statement in function returning non-void
  CC      arch/arm/mach-omap2/prm44xx.o
  CC      arch/arm/mach-omap2/omap_hwmod.o
In file included from arch/arm/plat-omap/include/plat/omap_hwmod.h:38,
                 from arch/arm/mach-omap2/omap_hwmod.c:145:
arch/arm/plat-omap/include/plat/voltage.h: In function 'omap_voltage_register_pmic':
arch/arm/plat-omap/include/plat/voltage.h:137: warning: no return statement in function returning non-void
  CC      arch/arm/mach-omap2/omap_hwmod_common_data.o
In file included from arch/arm/plat-omap/include/plat/omap_hwmod.h:38,
                 from arch/arm/mach-omap2/omap_hwmod_common_data.c:20:
arch/arm/plat-omap/include/plat/voltage.h: In function 'omap_voltage_register_pmic':
arch/arm/plat-omap/include/plat/voltage.h:137: warning: no return statement in function returning non-void

The error is reported when omap2plus_defconfig built with CONFIG_PM disabled
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Thara Gopinath <thara@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>

arch/arm/plat-omap/include/plat/voltage.h

@@ -134,7 +134,10 @@ void omap_change_voltscale_method(struct voltagedomain *voltdm,
 int omap_voltage_late_init(void);
 #else
 static inline int omap_voltage_register_pmic(struct voltagedomain *voltdm,
-		struct omap_volt_pmic_info *pmic_info) {}
+		struct omap_volt_pmic_info *pmic_info)
+{
+	return -EINVAL;
+}
 static inline  void omap_change_voltscale_method(struct voltagedomain *voltdm,
 		int voltscale_method) {}
 static inline int omap_voltage_late_init(void)