cmd: fix gpt setenv

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

Having a partition table with a single partition 63 is valid.

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>