lora: sx12xx_common: thread-safe API

Ensure that the modem is not asked to perform new operations before the
previous operation completes.

An atomic variable is used instead of a mutex as multiple threads need
to release the lock. A semaphore isn't used as there is no indication
whether `k_sem_give` gives the semaphore or not, which is required to
determine if `Radio.Sleep()` should be run.

`Radio.Sleep()` is only ever run by the context that successfully
releases the modem usage, to guard against double calls.

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