USB: POWERPC: ehci: fix ppc build

Currently, this setup:
CONFIG_USB_ARCH_HAS_EHCI=y
CONFIG_USB_EHCI_HCD=y
CONFIG_USB_EHCI_HCD_PPC_OF=y

Will fail to build:
  CC      drivers/usb/host/ehci-hcd.o
drivers/usb/host/ehci-hcd.c:1018:2: error: #error "missing bus glue for ehci-hcd"
make[3]: *** [drivers/usb/host/ehci-hcd.o] Error 1

ehci-hcd.c actually contains OF_PLATFORM_DRIVER glue, so error is bogus.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>