i2c: aspeed: use proper annotation for "fall through"

Use a better annotation, so GCC won't complain anymore:

drivers/i2c/busses/i2c-aspeed.c:458:15: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Tested-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>