zephyr: flash map: fix comparison signedness

This resolves a warning when building with `-Wsign-compare`.

`struct flash_area.fa_size` is declared as `size_t` in the Zephyr source
tree (in `include/zephyr/storage/flash_map.h`).

Signed-off-by: Samuel Coleman <samuel.coleman@rbr-global.com>
(cherry picked from commit 8b7c7cc9)