usb: host: ehci-sys: delete useless bus_to_hcd conversion

commit 0521cfd0 upstream.

The ehci platform device's drvdata is the pointer of struct usb_hcd
already, so we doesn't need to call bus_to_hcd conversion again.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: Unfortunately some EHCI platform sub-drivers 
 point drvdata to a private structure, so only create and remove the
 attributes if drvdata has been set as expected.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>