display_options: Do not use %llu in print_size

tiny-printf variant doesn't know how to handle %llu format string, but both
tiny-printf and print_size can meet in SPL when TFTP is used to obtain main
u-boot image. This is known to lead to critical boot issue at AM335x platform
when printf is catched in infinite loop.

To avoid such issues and make print_size function tiny-printf friendly, use %u
instead of %luu. Note, that the size value is guaranteed to be less than 1024
in this conditional branch, so the cast to unsigned int is safe.

Signed-off-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>