Avoid evaluating the XHCI control endpoint context.

The XHCI specification says that the XHCI controller should detect
reception of the USB device descriptors, and automatically update
the max packet size in the control endpoint context.

Differential Revision: https://reviews.freebsd.org/D26104
Reviewed by: kp@
MFC after: 1 week
Sponsored by: Mellanox Technologies