realtek: clear spurious GPIO interrupts

realtek: clear spurious GPIO interrupts

The interrupt controller in the internal GPIO peripheral will sometimes
generate spurious interrupts. If these are not properly acknowledged, the
system will be held busy until reboot. These spurious interrupts are identified
by the fact that there is no system IRQ number associated, since the interrupt
line was never allocated. Although most prevalent on RTL839x, RTL838x SoCs have
also displayed this behaviour.

Reported-by: Luiz Angelo Daros de Luca <luizluca@gmail.com> # DGS-1210-52
Reported-by: Birger Koblitz <mail@birger-koblitz.de> # Netgear GS724TP v2
Reported-by: Jan Hoffmann <jan@3e8.eu> # HPE 1920-16G
Signed-off-by: Sander Vanheule <sander@svanheule.net>