dm: usb: Avoid exceeding available array size for storage devices

The limit on storage devices is USB_MAX_STOR_DEV but we use one extra
element while probing to see if a device is a storage device. Avoid this,
since it causes memory corruption.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Stephen Warren <swarren@nvidia.com>