pckbd: add state variable for interrupt source

Currently there is only one flag to distinguish between two
interrupt sources and there are no available flags for more
sources. Add an internal state variable to store the interrupt
source. The next patch will introduce an additional interrupt
source. There is no functional change.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20210525181441.27768-7-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>