EHCI: force high-speed devices to run at full speed

This patch (as710) adds a sysfs class-device attribute file named
"companion" for EHCI controllers.  The file contains a list of port
numbers that are dedicated to the companion controller; by writing a
port number to the file the user can force a high-speed device
attached directly to the computer to run at full speed.  (As far as I
know it is not possible to do this for a device attached to an
external hub.)  A port is removed from the file by writing the
negative of its port number.

Several users have asked for this facility and it seems like a useful
thing to have.  Every now and then one runs across a device which
behaves much better at full speed than at high speed.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>