pp5002: Fix warning in the USB detection code

A function that returned bool was an alias for a function that returned
an int. While that original function was limited to returning either 0
or 1, this aliasing is technially a no-no.  So create a small shim to
make the warning go away.

Change-Id: I4d7807730234928bd59d75f13a4e4adeabbf655e

firmware/target/arm/pp/usb-fw-pp5002.c

@@ -78,4 +78,8 @@ int usb_detect(void)
 
     return USB_EXTRACTED;
 }
-bool usb_plugged(void) __attribute__((alias("usb_detect")));
+
+bool usb_plugged(void)
+{
+    return (usb_detect() == USB_INSERTED);
+}