Blackfin: gptimers: fix thinko when disabling timers

We only want to clear the run bit for this one timer, not all status bits.
So don't read the whole reg and then write all the bits back out.

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