Blackfin: time-ts: ack gptimer sooner to avoid missing short ints

If the period of a gptimer is fairly low, we might miss an interrupt
by acking it too late (we end up acking the new int as well).

Reported-by: Isabelle Leonardi <i.leonardi@detracom.fr>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>