Merge remote-tracking branch 'remotes/kraxel/tags/pull-usb-20170614-1' into staging

usb: bugfixes for ehci and xhci

# gpg: Signature made Wed 14 Jun 2017 08:44:01 BST
# gpg:                using RSA key 0x4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/pull-usb-20170614-1:
  ehci: stop recursive calls to ehci_work_bh
  xhci: only update dequeue ptr on completed transfers

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>