usb: dwc3: make dwc3_debugfs_init return value be void

Debugfs init failure is not so important. We can continue our job on
this failure. Also no break need for debugfs_create_file call failure.

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

[felipe.balbi@linux.intel.com :
	- remove out-of-memory message, we get that from OOM.
	- switch dev_err() to dev_dbg() ]

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