iavf: fix double-release of rtnl_lock

This code does not jump to exit on an error in iavf_lan_add_device(),
so the rtnl_unlock() from the normal path will follow.

Fixes: b66c7bc1 ("iavf: Refactor init state machine")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>