i2c: stm32: always call runtime_get when registering targets

Current code only calls pm_device_runtime_get when registering target
devices if the device is marked as wakeup capable by the application.

This does quite work for a setup with PM=n and PM_DEVICE=y and
PM_DEVICE_RUNTIME=y, where the CPU does not really go in stop mode ever,
and results in the i2c target device not working at all.

Fix this by always claiming the device when PM_DEVICE_RUNTIME is
enabled.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>