usb: dwc3: fix missing platform_set_drvdata() in dwc3_of_simple_probe()

Add missing platform_set_drvdata() in dwc3_of_simple_probe(), otherwise
calling platform_get_drvdata() in remove returns NULL.

This is detected by Coccinelle semantic patch.

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>