Use rgmii-id phy mode for the CPU port (MAC0) of the QCA8334 switch
to add delays to both Tx and Rx clock.

It worked with the rgmii mode before because the qca8k driver
(incorrectly) enabled delays in that mode and rgmii-id was not
implemented at all.

Commit 5ecdd77c ("net: dsa: qca8k: disable delay for RGMII mode")
removed the delays from the RGMII mode and hence broke the networking.

To fix the problem, commit a968b5e9 ("net: dsa: qca8k: Enable delay
for RGMII_ID mode") was introduced.

Now the correct phy mode is available so use it.
Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>