fastboot: Fix overflow when calculating chunk size

If a chunk was larger than 4GiB, then chunk_data_sz would overflow and
blkcnt would not be calculated correctly. Upgrade it to a u64 and cast
its multiplicands as well. Also fix bytes_written while we're at it.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Heiko Schocher <hs@denx.de>