powerpc/hvc_udbg: Don't crash when udbg_putc is NULL

Also while at it, add some help text indicating why you shouldn't
enable that driver under normal circumstances
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

drivers/tty/hvc/Kconfig

@@ -71,6 +71,10 @@ config HVC_UDBG
        depends on PPC && EXPERIMENTAL
        select HVC_DRIVER
        default n
+       help
+         This is meant to be used during HW bring up or debugging when
+	 no other console mechanism exist but udbg, to get you a quick
+	 console for userspace. Do NOT enable in production kernels. 
 
 config HVC_DCC
        bool "ARM JTAG DCC console"

drivers/tty/hvc/hvc_udbg.c

@@ -36,7 +36,7 @@ static int hvc_udbg_put(uint32_t vtermno, const char *buf, int count)
 {
 	int i;
 
-	for (i = 0; i < count; i++)
+	for (i = 0; i < count && udbg_putc; i++)
 		udbg_putc(buf[i]);
 
 	return i;
@@ -67,6 +67,9 @@ static int __init hvc_udbg_init(void)
 {
 	struct hvc_struct *hp;
 
+	if (!udbg_putc)
+		return -ENODEV;
+
 	BUG_ON(hvc_udbg_dev);
 
 	hp = hvc_alloc(0, NO_IRQ, &hvc_udbg_ops, 16);
@@ -88,6 +91,9 @@ module_exit(hvc_udbg_exit);
 
 static int __init hvc_udbg_console_init(void)
 {
+	if (!udbg_putc)
+		return -ENODEV;
+
 	hvc_instantiate(0, 0, &hvc_udbg_ops);
 	add_preferred_console("hvc", 0, NULL);