usb: gadget: f_acm: make bInterfaceProtocol configurable

usb: gadget: f_acm: make bInterfaceProtocol configurable

The bInterfaceProtocol is hardcoded to USB_CDC_ACM_PROTO_AT_V25TER. This
will lead to problems with ModemManger which will gladly try to probe
that port as a modem if the gadget also has a network function.
ModemManager will try to send AT commands to the ACM port. Make the
bInterfaceProtocol configurable. For this, track the number of instances
and only allow write to the property if there are no intances (yet).

This will also set bFunctionProtocol to the same value, see commit
5c8db070b448 ("USB: Change acm_iad_descriptor bFunctionProtocol to
USB_CDC_ACM_PROTO_AT_V25TER") for more details.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/20240825180446.3757073-1-mwalle@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>