cmd: usb: ignore blk, emulation devices in usb tree/info display

Usb tree/info commands iterate over all usb uclass devices recursively.
Blk uclass devices based on struct blk_desc are created for mass storage
device, treating them as usb uclass devices based on struct usb_device
and referencing usb config interface descriptors cause crash.
To fix, ignore blk and usb_emul uclass devices in usb_show_info
and usb_tree_graph. Also avoid addition of preamble for blk uclass
child devices, otherwise tree dump gets messed up.

Signed-off-by: Suneel Garapati <suneelglinux@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>