[MIPS] Fix warning about init_initrd() call if !CONFIG_BLK_DEV_INITRD.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index fdbb508..8f6e896 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -223,7 +223,11 @@
 
 #else  /* !CONFIG_BLK_DEV_INITRD */
 
-#define init_initrd()		0
+static unsigned long __init init_initrd(void)
+{
+	return 0;
+}
+
 #define finalize_initrd()	do {} while (0)
 
 #endif