usb: chipidea: fix the build error with randconfig

usb: chipidea: fix the build error with randconfig

Using below  configs, the compile will have error:
ERROR: "ehci_init_driver" undefined!

.config:
CONFIG_USB_CHIPIDEA=m
CONFIG_USB_CHIPIDEA_HOST=y
CONFIG_USB_CHIPIDEA_DEBUG=y

The reason is chipidea host uses symbol from ehci, but ehci
is not compiled. Let the chipidea host depend on
ehci even it is built as module.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>