hw/pl031: Actually raise interrupt on timer expiry

hw/pl031: Actually raise interrupt on timer expiry

Fix a typo in pl031_interrupt() which meant we were setting a bit
in the interrupt mask rather than the interrupt status register
and thus not actually raising an interrupt. This fix allows the
rtctest program from the kernel's Documentation/rtc.txt to pass
rather than hanging.

Reported-by: Daniel Forsgren <daniel.forsgren@enea.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Andreas Färber <afaerber@suse.de>