eatt: Fix getting channel with the queue data to send

When GATT is asking about channel with a non empty data queue to send,
EATT shall check also if the outstanding command is scheduled to send or
maybe is already waiting for the response.
Otherwise, we might end up with having channel not beeing used.

Bug: 249992170
Test: atest BluetoothInstrumentationTests
Test: atest --host net_test_eatt
Tag: #feature
Change-Id: Idceaf76847851cc8e256da4a3c4069c9e5cc93fd