stm32cube: stm32wl: Fix LL_SYSCFG_GetEXTISource

This function was not returning the expected port.
Fix it

(cherry picked from commit 44cc276a484cf42536442b563f5f666ce96b987f)

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
GitOrigin-RevId: 44cc276a484cf42536442b563f5f666ce96b987f
Change-Id: Ie8bfb71e8d3a7e651d0241298d3dc19bb4bfc68b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/hal_stm32/+/3424186
Tested-by: CopyBot Service Account <copybot.service@gmail.com>
Reviewed-by: Aaron Massey <aaronmassey@google.com>
Commit-Queue: Aaron Massey <aaronmassey@google.com>
Tested-by: Aaron Massey <aaronmassey@google.com>