cmd_i2c: change variable type for 10bit addressing support

To store 10bit chip address, the variable type should not be uchar,
but uint.

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