Distinguish between size and capacity

[Ivan-Velickovic]

For the various device class queue APIs. We use 'size' in APIs such as net_queue_size to mean how many entries are in the queue right now. But, we also use size to mean the capacity of the queue.

This is confusing and so we should change the terminology in all of the device classes. We should update the sDDF design doc as well if necessary.

• I2C
• Serial
• Network
• Block
• Sound