usb: sierra: fix a missing check of device_create_file

device_create_file() could fail and return an error code. The fix
captures the error and returns the error code upstream in case it
indeed failed.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>