net: dsa: sja1105: Increase priority of CPU-trapped frames

Without noticing any particular issue, this patch ensures that
management traffic is treated with the maximum priority on RX by the
switch.  This is generally desirable, as the driver keeps a state
machine that waits for metadata follow-up frames as soon as a management
frame is received.  Increasing the priority helps expedite the reception
(and further reconstruction) of the RX timestamp to the driver after the
MAC has generated it.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>