usb_cdc: export receive window for host flow control

[Argolein]

This PR matches the behavior already used by the UART and CAN serial transports, which expose their MCU-side receive window to the host so that Klipper can throttle outstanding command data appropriately.

Hardware / Reproduction Setup

This was tested on:

• Duet3D Mini 5+ with SAME54P20A
• USB CDC connection to the Klipper host
• Raspberry Pi 4B running Klipper / Moonraker
• EBB36 Tool Board connected with USB as well

Issue I had:

During normal prints, the main MCU connection would sporadically enter phases where bytes_retransmit increased rapidly and eventually caused a shutdown. bytes_invalid stayed at zero, and the EBB connection did not show the same behavior.

After fix applied:

After exporting RECEIVE_WINDOW for USB CDC, the same print workload passed the previous retransmit escalation window without the main MCU retry counter rising.

[Argolein]

-deleted not relevant-

[github-actions]

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