Bluetooth: TBS: Replace busy bool with atomic

Replace the busy boolean flag with an atomic value.
This prevents any race conditions with the implementation.

The discovery procedure is also now properly guarded with it
so that in case that any procedure is currently in progress
for the specific connection, then a new discovery procedure
cannot happen until all other procedures are finished.

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