usb: dwc3: fix extcon dependency

The dwc3 core support now links against the extcon subsystem,
so it cannot be built-in when extcon is a loadable module:

arm-linux-gnueabi-ld: drivers/usb/dwc3/core.o: in function `dwc3_get_extcon':
core.c:(.text+0x572): undefined reference to `extcon_get_edev_by_phandle'
arm-linux-gnueabi-ld: core.c:(.text+0x596): undefined reference to `extcon_get_extcon_dev'
arm-linux-gnueabi-ld: core.c:(.text+0x5ea): undefined reference to `extcon_find_edev_by_node'

There was already a Kconfig dependency in the dual-role support,
but this is now needed for the entire dwc3 driver.

It is still possible to build dwc3 without extcon, but this
prevents it from being set to built-in when extcon is a loadable
module.

Fixes: d182c2e1

 ("usb: dwc3: Don't switch OTG -> peripheral if extcon is present")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20230118090147.2126563-1-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>