Rename SERIAL_PROXY_MODE_EZSP_ASH to SERIAL_PROXY_MODE_PROTOCOL so the
serial_proxy API surface carries no protocol-specific names. The mode now
means "the port's tap is active"; which protocol the tap speaks is a
property of the device configuration, discoverable from the tap
component's own API surface. Future protocol taps need no serial_proxy
or API changes.
The usb_uart_id key was redundant: uart_id already points at the channel.
A new usb_uart.is_usb_uart_channel() helper checks the config tree (use_id
resolution does not narrow the ID type), and zigbee_proxy uses it to enable
the RX callback fast path and USB timeout defaults automatically.
kbx81