MPC5xxx: fix some compiler warnings in USB code

Fix the following warnings:
- usb.c:xx: warning: function declaration isn't a prototype
- usb_ohci.c:xxx: warning: passing argument 1 of '__fswab32' makes integer
  from pointer wihtout a cast

Signed-off-by: Martin Krause <martin.krase@tqs.de>
diff --git a/cpu/mpc5xxx/usb.c b/cpu/mpc5xxx/usb.c
index ce709fc..ed467ab 100644
--- a/cpu/mpc5xxx/usb.c
+++ b/cpu/mpc5xxx/usb.c
@@ -27,7 +27,7 @@
 
 #include <mpc5xxx.h>
 
-int usb_cpu_init()
+int usb_cpu_init(void)
 {
 	/* Set the USB Clock						     */
 	*(vu_long *)MPC5XXX_CDM_48_FDC = CONFIG_USB_CLOCK;
@@ -41,12 +41,12 @@
 	return 0;
 }
 
-int usb_cpu_stop()
+int usb_cpu_stop(void)
 {
 	return 0;
 }
 
-int usb_cpu_init_fail()
+int usb_cpu_init_fail(void)
 {
 	return 0;
 }