usbdi(9): Restore usb_descriptor_t to its correct definition.

Descriptors in the USB spec all start with bLength and
bDescriptorType.  bDescriptorSubtype is only for certain
class-specific descriptors.  Many descriptors, such as
usb_device_descriptor_t, _do not_ have bDescriptorSubtype, so using a
structure that has bDescriptorSubtype for such descrpitors is wrong.

There is some history here:

- Back in 1998, when augustss@ introduced the USB stack, the type
  usb_descriptor_t was erroneously defined with a bDescriptorSubtype
  member.

- In 2007, drochner@ removed this member to accurately reflect the
  USB spec.

- In 2018, khorben@ appeared to have accidentally reintroduced it
  while importing the umb(4) driver from OpenBSD, which still has the
  erroneous bDescriptorSubtype member in usb_descriptor_t.

The umb(4) driver has since been adjusted to correctly use
usb_cdc_descriptor_t instead of usb_descriptor_t (and I have now
restored umidi_cs_descriptor_t which I had removed last month before
I realized this history of usb_descriptor_t), so this member is no
longer necessary.

(riastradh)