Fix issue with Linux guest XHCI tablet probing.

The USB3 spec mandates that the device-descriptor max packet size
be 512 bytes, which requires a field size of 9 since it is a
power-of-2.

Linux kernels recently started validating this field, resulting in
the table not being probed and the cursor not working in bhyve VNC.

Reviewed by: corvink
PR: 275760
MFC after: 1 week