ALSA: usb-audio: Guard Scarlett2 protocol transfers

Scarlett2 controls and hwdep operations reach the proprietary USB
transport without preventing runtime suspend or disconnect. Protect the
central request-and-response helper. One reference then covers the
command, acknowledgment wait, and response.

The runtime and system resume hook submits only the notification URB. It
takes no protocol mutex. Thus, the transport guard does not invert the
existing data_mutex or usb_mutex order. Keep the suspend-time config save
on the unguarded helper because it runs inside the USB suspend callback.

This change protects each USB transaction. It does not hold a runtime-PM
reference across the asynchronous flash-erase interval.

Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Will Porter <mrwillporter@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20260827232143.149197-5-mrwillporter@gmail.com

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