applications: nrf5340_audio: Prevent hard coding frame duration in LE_AUDIO_SDU_SIZE_OCTETS

[Cyrillius]

LE_AUDIO_SDU_SIZE_OCTETS is a define which is used in nrf aura config application inside the function bt_le_audio_tx_send (https://github.com/nrfconnect/sdk-nrf/blob/v3.1.0/applications/nrf5340_audio/src/bluetooth/bt_stream/bt_le_audio_tx/bt_le_audio_tx.c#L259) with a frame duration other than 10ms like 7.5ms.

The problem is that this define is using an hard coded value of the frame duration and will miscalculate from the bitrate the sdu size and fail to send data.

Another solution would be to stop using LE_AUDIO_SDU_SIZE_OCTETS and let it as it is for the nrf5340 audio demo application which is using hard coded configuration.

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