i2c: Correct command return values

We should not return a -ve error code from command functions. Instead,
return CMD_RET_FAILURE. This avoids the "exit not allowed from main input
shell" error messages from the hush shell.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>