usb: ulpi: add indicator configuration function

Allows for easy configuration of the VBUS indicator related ULPI
config bits.

Also move the external indicator setup from ulpi_set_vbus() to
the new function.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>