Bluetooth: BAP: Shell: Improve handling of PSN when sending

Previously the calculation of the PSN was done using
just a timer. This would not work correctly when
attempting to send multiple packets in a single SDU
interval (e.g. to enqueue 2 or more).

Previously we only ever attempted to send 1 packet in
the `sent` callback but scheduling the work item. However
in the case that the `sent` callback was called twice
before the work item had been triggered (possible due
to the priority of the RX thread being higher than the
system workqueue thread). This has been modified so that
if we can enqueue more packets in the lc3_audio_send_data
function, we re-enqueue the work item. We re-enqueue rather
than sending multiple in a single call to avoid blocking
other items on the system workqueue from being scheduled.
This could cause possible missed intervals (if other workqueue
items are processed), but it should be OK from a ISO perspective,
and the new PSN calculator handles this much better.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>