cmd: let gpt_partition_entry be hexadecimal

In commands like 'ls mmc 0:f' the partition number is hexadecimal.

In command 'gpt setenv' variable gpt_partition_entry needs to be set
to a hexadecimal value to allow its use as a parameter in a
subsequent command.

Fixes: 57f8cf1b9aea ("cmd: fix gpt enumerate")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>