usb: dwc3: fix memory leak of dwc->regset

dwc->regset is allocated on dwc3_debugfs_init, and should
be released on init failure or dwc3_debugfs_exit. Btw,
The line "dwc->root = NULL" is unnecessary, so remove it.

Signed-off-by: Du, Changbin <changbin.du@intel.com>

[ felipe.balbi@linux.intel.com : add another err label for the new
	error condition ]

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