Due to the IRQ processing, we can generate another IRQ that can come before we
end the previous one, so we lost it. E.g. when transmitting a character.

To allow the processing in SMP machines, we ack the IRQ at the beginning of the
IRQ handler.
Signed-off-by: NSamuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>