Rename i2c_commands.c to i2c_common.c

It will soon have cross-platform code moved to it.

This CL just does the renaming; no functional changes.

BUG=chrome-os-partner:18969
BRANCH=none
TEST=build spring,link

Change-Id: I8cc475f9dd1422a0a9c586c93eb0d7aaf21a001d
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/49359
Reviewed-by: Bill Richardson <wfrichar@chromium.org>