usb: gadget: f_uac2: fix AC Interface Header Descriptor wTotalLength

As per UAC2 spec (ch. 4.7.2), wTotalLength of AC Interface
Header Descriptor "includes the combined length of this
descriptor header and all Clock Source, Unit and Terminal
descriptors."

Thus add its size to its wTotalLength.

Also after recent changes wTotalLength is calculated
dynamically, update static definition of uac2_ac_header_descriptor
accordingly

Fixes: 132fcb46 ("usb: gadget: Add Audio Class 2.0 Driver")
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>