serial: 8250, disable "too much work" messages

The 8250 driver now reports many of these:
  serial8250: too much work for irq4
These messages turned out to be common these days with a use of
virtualization. I tried to increase the limit of processed characters
in commit e7328ae1848966181a7ac47e8ae6cddbd2cf55f3 (serial: 8250,
increase PASS_LIMIT) in 2011. It was raised from 256 to 512, but it is
still not enough, apparently.

So disable the warning unless somebody turns on DEBUG (or
DYNAMIC_DEBUG _and_ the message).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Martin Pluskal <mpluskal@suse.com>
Reported-by: Takashi Iwai <tiwai@suse.com>
Tested-by: Takashi Iwai <tiwai@suse.de>
References: https://bugzilla.novell.com/show_bug.cgi?id=868394
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_core.c

@@ -1601,8 +1601,7 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
 		l = l->next;
 
 		if (l == i->head && pass_counter++ > PASS_LIMIT) {
-			/* If we hit this, we're dead. */
-			printk_ratelimited(KERN_ERR
+			pr_debug_ratelimited(
 				"serial8250: too much work for irq%d\n", irq);
 			break;
 		}