USB: gpio_vbus: avoid consecutive vbus_session calls with the same "is_active"

Basically, ->vbus_session() calls should be served when VBUS session
starts and ends (it's not whenever transciever drivers detect VBUS
_changes_).  Otherwise, if UDC gadget drivers don't want for some
reason ->vbus_session() calls with the same "is_active" value, either
OTG or UDC drivers need to have some protection handlings.

Also, on platforms using this 'gpio_vbus' driver, the driver is only
allowed to check whether VBUS is applied.  There is no kernel-standard
way prepared for UDC gadget drivers to do that.

With this in mind, gpio_vbus should try to prevent unnecessary
consecutive vbus_session calls being served with the same "in_active"
value.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi.px@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>