Merge remote-tracking branch 'remotes/kraxel/tags/usb-20180821-pull-request' into staging

usb: mtp write support, bugfixes.

# gpg: Signature made Tue 21 Aug 2018 10:11:36 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# 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/usb-20180821-pull-request:
  dev-mtp: rename x-root to rootdir
  dev-mtp: Add support for > 4GB file transfers
  dev-mtp: retry write for incomplete transfers
  dev-mtp: fix buffer allocation for writing file contents
  dev-mtp: add support for canceling transaction
  ohci: Clear the interrupt counter for erroneous transfers
  docs/usb2.txt: ehci has six ports

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