usb, chipidea: fix link error when USB_EHCI_HCD is a module

Fixes link error when USB_EHCI_HCD=m and USB_CHIPIDEA_HOST=y:

	drivers/built-in.o: In function `ci_hdrc_host_init':
	drivers/usb/chipidea/host.c:104: undefined reference to `ehci_init_driver'

as a result of commit 09f6ffde ("USB: EHCI: fix build error by making
ChipIdea host a normal EHCI driver").

Cc: stable@vger.kernel.org [v3.7+]
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>