Skip enclosure service devices when probing for usb storage devices.

This avoids long timeouts when probing for external usb harddisks
which provide "Enclosure Services".

Signed-off-by: Soeren Moch <smoch@web.de>
--

This is a new version of the patch
"usb_storage: skip all unknown devices when probing"
http://http://lists.denx.de/pipermail/u-boot/2014-November/194622.html

Cc: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>