Starting with commit 1c380f94 dma
transfers can actually fail.  This patch makes ehci keep track
of the busmaster bit in pci config space, by setting/clearing the
dma_context pointer.  Attempts to dma without context will result
in raising HSE (Host System Error) interrupt and stopping the host
controller.

This patch fixes WinXP not booting with a usb stick attached to ehci.
Root cause is seabios activating ehci so you can boot from the stick,
and WinXP clearing the busmaster bit before resetting the host
controller, leading to ehci actually trying dma while it is disabled.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>