lora: detect failed transmissions

Detect failed transmissions using `lora_send` by only waiting for some
multiple of the actual on-air time before giving up.

We use this instead of the inbuilt TxTimeout functionality because the
value of this timeout is set by `lora_configure`, and therefore doesn't
change with different packet lengths. This is a limitation of the
underlying `RadioSend` function, not the Zephyr driver.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>