USB: gadget: ethernet error path potential oops fix

Fix potential (never-observed) oops on rare error path,
bugzilla #9594.  Fix uses the same test as used earlier.

Also make the adjacent "else" block look like an "else" block
instead of hiding like a bug.

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