lorawan: enforce larger system work queue stack size via Kconfig

The LoRaWAN subsystem uses the system work queue internally and needs
more than the default stack size of 1024 bytes. This is considered
in the sample application, but may be forgotten by out of tree users.

Enforcing it via Kconfig prevents users from accidentally getting
hard-to-debug stack overflows.

Signed-off-by: Martin Jäger <martin@libre.solar>