i2c: amd756: Remove superfluous TODO

This old driver has never been used on big-endian systems, so remove the
todo.

Suggested-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Atharva Tiwari <evepolonium@gmail.com>
[wsa: reworded commit message]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>