serial: 8250, increase PASS_LIMIT

With virtual machines like qemu, it's pretty common to see "too much
work for irq4" messages nowadays. This happens when a bunch of output
is printed on the emulated serial console. This is caused by too low
PASS_LIMIT. When ISR loops more than the limit, it spits the message.

I've been using a kernel with doubled the limit and I couldn't see no
problems. Maybe it's time to get rid of the message now?

[2.6.32: background info from Ram Gupta]

> I need a patch for serial driver that increases PASS_LIMIT merged in
> 3.1. I am using 2.6.32 kernel which experiences kernel panic
> occasionally. It will be great if you can backport to 2.6.32 and 3.0
> kernel. The commit ID is e7328ae1  serial: 8250, increase PASS_LIMIT
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
(cherry picked from commit e7328ae1)
Cc: Ram Gupta <ram.gupta5@gmail.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>

drivers/serial/8250.c

@@ -81,7 +81,7 @@ static unsigned int skip_txen_test; /* force skip of txen test at init time */
 #define DEBUG_INTR(fmt...)	do { } while (0)
 #endif
 
-#define PASS_LIMIT	256
+#define PASS_LIMIT	512
 
 #define BOTH_EMPTY 	(UART_LSR_TEMT | UART_LSR_THRE)