usb: mtu3: fix EXTCON dependency

When EXTCON is a loadable module, mtu3 fails to link as built-in:

drivers/usb/mtu3/mtu3_plat.o: In function `mtu3_probe':
mtu3_plat.c:(.text+0x690): undefined reference to `extcon_get_edev_by_phandle'

Add a Kconfig dependency to force mtu3 also to be a loadable module
if extconn is, but still allow it to be built without extcon.

Fixes: d0ed062a

 ("usb: mtu3: dual-role mode support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>