usb: gadget: composite: Inform controller driver of self-powered

Different configuration/condition may draw different power. Inform the
controller driver of the change so it can respond properly (e.g.
GET_STATUS request). This fixes an issue with setting MaxPower from
configfs. The composite driver doesn't check this value when setting
self-powered.

Cc: stable@vger.kernel.org
Fixes: 88af8bbe

 ("usb: gadget: the start of the configfs interface")
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>