serial8250: Mark console as CON_ANYTIME

While trying to debug a cpu-hotplug issue I noticed printk() stopped
working once the cpu got marked offline, since the 8250 serial console
doesn't have any per-cpu resources the CON_ANYTIME bit is the safe and
documented way to make it work again.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>