This fixes an issue where we dropped the send buffer when
ipc_service_send() timed out waiting for a shared memory buffer.
We now wait in the IPC RX endpoint, but this shouldn't be a problem for
other IPC users, as PR #43806 introduces separate workqueues
per instance, with configurable priorities.
This also includes small changes regarding the IPC service return value.
Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
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