serial8250: ratelimit "too much work" error

Running a serial console, if too many kernel messages are generated within
a short time causing a lot of serial I/O, the 8250 driver will generate
another kernel message reporting this, which just adds to the I/O. It has
a cascading effect and quickly results the system being brought to its knees
by a flood of "too much work" messages.

Ratelimit the error message to avoid this.

[akpm@linux-foundation.org: use the superior printk_ratelimited()]
[akpm@linux-foundation.org: printk_ratelimited() needs ratelimit.h]
Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>