hw/block/nand: Have blk_load() take unsigned offset and return boolean

hw/block/nand: Have blk_load() take unsigned offset and return boolean

Negative offset is meaningless, use unsigned type.
Return a boolean value indicating success.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240409135944.24997-3-philmd@linaro.org>
(cherry picked from commit 2e3e09b368001f7eaeeca7a9b49cb1f0c9092d85)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>