usb: fix first descriptor fetch error handling

When fetching the first descriptor from a new device, only validate that
we received at least 8 bytes, not that we received the entire descriptor.
The reasoning is:
- The code only uses fields in the first 8 bytes, so that's all we need
  to have fetched at this stage.
- The smallest maxpacket size is 8 bytes. Before we know the actual
  maxpacket the device uses, the USB controller may only accept a single
  packet (see the DWC2 note in the comment added in the commit).
  Consequently we are only guaranteed to receive 1 packet (at least 8
  bytes) even in a non-error case.

Fixes: 1a7758044b04 ("usb: Early failure when the first descriptor read
fails or is invalid")
Cc: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>