irqchip: or1k-pic: Undefine mask_ack for level triggered hardware

The mask_ack operation clears the interrupt by writing to the PICSR
register.  This we don't want for level triggered interrupt because
it does not actually clear the interrupt on the source hardware.

This was causing issues in qemu with multi core setups where
interrupts would continue to fire even though they had been cleared in
PICSR.

Just remove the mask_ack operation.

Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>