[serial_proxy] Make port mode protocol-neutral

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.

tests/components/zigbee_proxy/common.yaml

@@ -20,4 +20,4 @@ serial_proxy:
     uart_id: zigbee_uart
     name: Zigbee
     port_type: TTL
-    mode: ezsp_ash
+    mode: protocol

tests/components/zigbee_proxy/test-usb.esp32-s3-idf.yaml

@@ -22,7 +22,7 @@ serial_proxy:
     uart_id: zigbee_usb_channel
     name: Zigbee
     port_type: TTL
-    mode: ezsp_ash
+    mode: protocol
 
 zigbee_proxy:
   serial_proxy_id: zigbee_usb_serial