USB storage count

Here's another USB storage patch. Currently U-Boot handles storage
devices #0 - #4 as valid devices, even if there is none connected. This
patch fixes usb_stor_get_dev() to check detected device count instead
of MAX-define.

This is very important for ill behaving devices. usb_dev_desc[] can be
partially initialized if device probe fails.

After fixing get_dev() it was easy to fix "usb part" etc commands.
Previously it outputed "Unknown partition table" five times, now it's
"no USB devices available".

Signed-off-by: Kim B. Heino <Kim.Heino@bluegiga.com>