cmd: ide: Make the first device the default one

At present the IDE device number is initialized to -1, which means
we cannot type "ide read" command before setting the device number
via "ide device #".

For convenience, let's set the first device as the default one.

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