usbnet: init fault (oops) cleanup, whitespace fixes

This cleans up some error handling paths in usbnet device probing;
one of them could cause oopsing, e.g. with some RNDIS devices.

It also removes some extraneous whitespace.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>