powerpc/udbg: Make it safe to call udbg_printf() always

Make udbg_printf() check if udbg_putc is set, and if not just return.
This makes it safe to call udbg_printf() anytime, even when a udbg
backend has not been registered, which means we can avoid some ifdefs
at call sites.
Signed-off-by: Qian Cai <cai@lca.pw>
[mpe: Split out of larger patch, write change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/udbg.c

@@ -120,13 +120,15 @@ int udbg_write(const char *s, int n)
 #define UDBG_BUFSIZE 256
 void udbg_printf(const char *fmt, ...)
 {
-	char buf[UDBG_BUFSIZE];
-	va_list args;
+	if (udbg_putc) {
+		char buf[UDBG_BUFSIZE];
+		va_list args;
 
-	va_start(args, fmt);
-	vsnprintf(buf, UDBG_BUFSIZE, fmt, args);
-	udbg_puts(buf);
-	va_end(args);
+		va_start(args, fmt);
+		vsnprintf(buf, UDBG_BUFSIZE, fmt, args);
+		udbg_puts(buf);
+		va_end(args);
+	}
 }
 
 void __init udbg_progress(char *s, unsigned short hex)