usb: dwc3: pci: Runtime resume child device from wq

Driver currently resumes and increments pm usage_count
of its child device (dwc3 main) from its runtime_resume
handler. This requires dwc3 runtime_resume to perform
pm_runtime_put to decrement the pm usage_count. However
runtime_put from dwc3 happens for non pci drivers
(e.g. dwc3-if-simple.c) as well which results in dwc3
pm usage_count becoming negative after couple of
runtime suspend resume iterations. Fix this by
performing runtime_get/put from dwc3-pci driver only
using workqueue.

Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>