USB: EHCI: remove Alchemy EHCI driver

The platform code has been converted to use the ehci-platform driver instead
thus obsoleting the ehci-au1xxx driver, which can be removed.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>