cdcacm: Add the interface assoc descriptor to the length

[bergzand]

Contribution description

The interface association descriptor length must be included in the descriptor length claimed by the descriptor generator, otherwise the USB stack will report an incorrect total descriptor length.

Testing procedure

Before this PR tests/usbus_cdc_acm_stdio should be either broken or the Linux kernel should report something along the lines of:

[415964.073466] usb 1-9.2.2: new full-speed USB device number 113 using xhci_hcd
[415964.151103] usb 1-9.2.2: config 1 has an invalid descriptor of length 7, skipping remainder of the config
[415964.151111] usb 1-9.2.2: config 1 interface 1 altsetting 0 has 0 endpoint descriptors, different from the interface descriptor's value: 2
[415964.151779] usb 1-9.2.2: New USB device found, idVendor=1209, idProduct=0001, bcdDevice= 0.00
[415964.151784] usb 1-9.2.2: New USB device strings: Mfr=3, Product=2, SerialNumber=0
[415964.151787] usb 1-9.2.2: Product: USB device
[415964.151790] usb 1-9.2.2: Manufacturer: RIOT-os.org
[415964.156518] cdc_acm: probe of 1-9.2.2:1.0 failed with error -22

This PR fixes that.

Issues/PRs references

Broken by #12430

[chrysn]

Confirming that the misbehavior was recently introduced in master, and is fixed by this patch. History confirmed on a custom board branch with the saul example (CDC-ACM only), tested also in combination with CDC-ECM in the gcoap example.

[miri64]

Proxy-ACK. You know better than I what you are doing ;-)