USB: Move usb_iso_packet_descriptor to a separate header.

The USB Kit uses it, so this allows the USB Kit to stop including
USB3.h.

Change-Id: Ifde025ec41bef92013fda0440d60b7216cfdbe4a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4413
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>