usb: chipidea: Handle extcon events properly

We're currently emulating the vbus and id interrupts in the OTGSC
read API, but we also need to make sure that if we're handling
the events with extcon that we don't enable the interrupts for
those events in the hardware. Therefore, properly emulate this
register if we're using extcon, but don't enable the interrupts.
This allows me to get my cable connect/disconnect working
properly without getting spurious interrupts on my device that
uses an extcon for these two events.

Acked-by: Peter Chen <peter.chen@nxp.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Ivan T. Ivanov" <iivanov.xz@gmail.com>
Fixes: 3ecb3e09 ("usb: chipidea: Use extcon framework for VBUS and ID detect")
Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Peter Chen <peter.chen@nxp.com>