usb: protocol field in descriptor for CDC ACM should default to zero

[IvanSanchez]

The protocol field here at

zephyr/subsys/usb/usb_descriptor.c

Line 207 in 4304b4c

.bInterfaceProtocol = V25TER_PROTOCOL,

should default to 0x00 (meaning "no protocol") instead of 0x01 (meaning "I'm actively advertising that I support the AT V.250 protocol" AKA V25TER).

Although this field has been ignored by major OSs during the last decades, this creates a big deal of problems with ModemManager sending unexpected AT commands when plugging in a dev board. This has been mitigated up to some point by having ID_MM_DEVICE_IGNORE udev rules all over the place, which is just a stopgap measure.

This is a simple change, but I don't want to send a pull request before more work in #4661 is done due to fear of merge conflicts. Also, some applications might want to advertise support for some AT command set or ethernet emulation or a vendor protocol (see table here), which is related to #4661 (refactor usb descriptors) and #6593 (let developers specify some values when putting together the usb descriptors).

cc @finikorg @jfischer-phytec-iot

[finikorg]

@IvanSanchez OK with the change proposed, are you going to send a PR?

[IvanSanchez]

@finikorg I'd rather wait for #4661.

[nashif]

@finikorg can you submit a PR to fix this, looks like #4661 will never happen!

[finikorg]

@nashif done