i2c: fix stack buffer overflow vulnerability in i2c md command

This reinstates fix from commit 8f8c04bf
without the changes unrelated to the actual fix. Avoid the underflow by
setting only nbytes and linebytes as unsigned integers.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Nicolas Iooss <nicolas.iooss+uboot@ledger.fr>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tim Harvey <tharvey@gateworks.com>
Acked-by: Tim Harvey <tharvey@gateworks.com>