USB: EHCI: remove IXP4xx EHCI driver

This driver is not registered by any in-tree user. If needed it the EHCI
driver can be reinstatied using the ehci-platform driver with caps_offset to
0x100.

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>