usb: renesas: fix extcon dependency

The renesas usbhs driver calls extcon_get_edev_by_phandle(), which
is defined in drivers/extcon/extcon-class.c, and that can be a
loadable module. If the extcon-class support is disabled, usbhs
will work correctly for all devices that do not need extcon.

However, if extcon-class is a loadable module, and usbhs is
built-in, the kernel fails to link. In order to solve that,
we need a Kconfig dependency that allows extcon to be disabled
but does not allow usbhs built-in if extcon is a module.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>