gpio: virtuser: fix handling of multiple conn_ids in lookup table

Creating a virtuser device via configfs with multiple conn_ids fails due
to incorrect indexing of lookup entries. Correct the indexing logic to
ensure proper functionality when multiple gpio_virtuser_lookup are
created.

Fixes: 91581c4b ("gpio: virtuser: new virtual testing driver for the GPIO API")
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Link: https://lore.kernel.org/r/20250103141829.430662-3-koichiro.den@canonical.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>