libpayload: Fix some missing-prototype warnings

usb_controller_initialize() is not declared in any header file nor
called from outside of usbinit.c, so make it static.

set_configuration() looks like beeing non-static on purpose (like the
other helpers around it in usb.c), so put a prototype into usb.h.

Change-Id: I08d93b3769d8398bb43462d9afdfeec81fef93ec
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: http://review.coreboot.org/1894
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
diff --git a/payloads/libpayload/drivers/usb/usbinit.c b/payloads/libpayload/drivers/usb/usbinit.c
index 4419ce6..f95e9c1 100644
--- a/payloads/libpayload/drivers/usb/usbinit.c
+++ b/payloads/libpayload/drivers/usb/usbinit.c
@@ -42,7 +42,7 @@
  * @param dev PCI device id at bus
  * @param func function id of the controller
  */
-int usb_controller_initialize(int bus, int dev, int func)
+static int usb_controller_initialize(int bus, int dev, int func)
 {
 	u32 class;
 	u32 devclass;