usb: misc small fixes.

# gpg: Signature made Wed 13 Sep 2017 10:28:25 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/usb-20170913-pull-request:
  usb: only build usb-host with CONFIG_USB=y
  usb: drop HOST_USB
  MAINTAINERS: add missing USB entry
  xhci: Avoid DMA when ERSTBA is set to zero

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