lorawan: Rework error handling code

For consistency with other parts of Zephyr, the public APIs available
in lorawan subsystem now returns error codes from the set defined in
errno.h.

Signed-off-by: Kuba Sanak <contact@kuba.fyi>
[mani: reworked the code and commit a bit for upstream]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>