usb: dwc3: gadget: Fix suspend/resume during device mode

Gadget controller might not be always active during system
suspend/resume as gadget driver might not have yet been loaded or
might have been unloaded prior to system suspend.

Check if we're active and only then perform
necessary actions during suspend/resume.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>