PCI: mediatek-gen3: Enable async probe by default

The mediatek-gen3 driver can run its probe routine fairly slow on some
hardware, which adds to the total time it takes for the system start up.

Thus, turn on async mode for the probe to avoid blocking the rest of the
system.

Link: https://lore.kernel.org/r/20241220145205.1.Ibf2563896c3b1fc133bb46d3fc96ad0041763922@changeid


Signed-off-by: Douglas Anderson <dianders@chromium.org>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>