i2c: fix stack buffer overflow vulnerability in i2c md command

This reinstates fix from commit 8f8c04bf ("i2c: fix stack buffer
overflow vulnerability in i2c md command") 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>