gpio: sprd: Fix incorrect irq type setting for the async EIC

When setting async EIC as IRQ_TYPE_EDGE_BOTH type, we missed to set the
SPRD_EIC_ASYNC_INTMODE register to 0, which means detecting edge signals.

Thus this patch fixes the issue.

Fixes: 25518e02

 ("gpio: Add Spreadtrum EIC driver support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Neo Hou <neo.hou@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>