CHROMIUM: qca8k: enable port flow control

Set phy device advertising to enable MAC flow control.

BUG=b:136488170
TEST=compile

Change-Id: Ibf0f554b072fc73136ec9f7ffb90c20b25a4faae
Signed-off-by: xiaofeis <xiaofeis@codeaurora.org>
Reviewed-on: https://chromium-review.googlesource.com/1708012


Tested-by: Zong-cing Lin <zclin@google.com>
Tested-by: Julan Hsu <julanhsu@google.com>
Commit-Ready: Julan Hsu <julanhsu@google.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Zong-cing Lin <zclin@google.com>
Reviewed-by: Julan Hsu <julanhsu@google.com>