cmd_i2c: quit I2C commands immediately on error

If the i2c driver returns an error status, error out immediately.
Continuing the loop just results in printing error messages
again and again.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>