MFC r271953:
Some XHCI hardware requires dropping the endpoint context before
adding it again.

Approved by:	re, glebius
MFC r271949:
Fix UNMAP stuck if the last block descriptor in the list is empty.

Approved by:	re (glebius)
MFC r271957:  Fix read overrun handling, broken by using wrong variable.

Approved by:	re (marius)