hw/intc/arm_gic: Fix GIC_SET_LEVEL

hw/intc/arm_gic: Fix GIC_SET_LEVEL

The GIC_SET_LEVEL macro unfortunately overwrote the entire level
bitmask instead of just or'ing on the necessary bits, causing active
level PPIs on a core to clear PPIs on other cores.

Cc: qemu-stable@nongnu.org
Reported-by: Rob Herring <rob.herring@linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Message-id: 1393031030-8692-1-git-send-email-christoffer.dall@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 6453fa998a11e133e673c0a613b88484a8231d1d)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>