usb: gadget: configfs: add suspend/resume

USB gadgets composed with configfs lack suspend and resume
methods. This patch uses composite_suspend()/composite_resume()
the same way e.g. composite_setup() or composite_disconnect()
are used in a configfs-based gadget.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>