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

@@ -120,6 +120,7 @@ int udbg_write(const char *s, int n)
 #define UDBG_BUFSIZE 256
 void udbg_printf(const char *fmt, ...)
 {
+	if (udbg_putc) {
 		char buf[UDBG_BUFSIZE];
 		va_list args;
 
@@ -127,6 +128,7 @@ void udbg_printf(const char *fmt, ...)
 		vsnprintf(buf, UDBG_BUFSIZE, fmt, args);
 		udbg_puts(buf);
 		va_end(args);
+	}
 }
 
 void __init udbg_progress(char *s, unsigned short hex)