USB: EHCI: Fix build warning in xilinx ehci driver

This fixes the following warning:
In file included from drivers/usb/host/ehci-hcd.c:1246:0:
drivers/usb/host/ehci-xilinx-of.c:293:2: warning: initialization from incompatible pointer type [enabled by default]
drivers/usb/host/ehci-xilinx-of.c:293:2: warning: (near initialization for 'ehci_hcd_xilinx_of_driver.shutdown') [enabled by default]

Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>