cmd: gpt: use UUID accessor more consistently

disk_partition_uuid() and disk_partition_set_uuid() were introduced to let
us avoid the usage of #ifdef when dealing with the field uuid of
struct disk_partition.

In allocate_disk_part() commit c5f1d005 ("part: Add accessors for
struct disk_partition uuid") missed to use the setter.

print_gpt_info() and create_gpt_partitions_list() are further functions
where we should use the getter.

Fixes: c5f1d005 ("part: Add accessors for struct disk_partition uuid")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>