platform/chrome: cros_ec: Fix leak in sequence_store()

The allocated cros_ec_command message structure is not freed in function
sequence_store(). Make sure that 'msg' is freed in all exit paths.
Detected by Coverity CID 1309667.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Olof Johansson <olof@lixom.net>