cmd: fix gpt enumerate

Do not assume that partitions are numbered continuously starting at 1.

Only a single partition table type can exist on a block device. If we found
a GPT partition table, we must not re-enumerate with the MBR partition
driver which would find the protective partition.

Fixes: 12fc1f3b ("cmd: gpt: add eMMC and GPT support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>