usb: fix ehci_hcd build failure when both generic-OF and xilinx is selected

This patch fixes the driver to allow both CONFIG_USB_EHCI_HCD_PPC_OF and
CONFIG_USB_ECHI_HCD_XILINX to be selected.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
CC: John Linn <john.linn@xilinx.com>
CC: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>