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

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.

Cc: <stable@vger.kernel.org>
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>