net: dsa: felix: unconditionally configure MAC speed to 1000Mbps

In VSC9959, the PCS is the one who performs rate adaptation (symbol
duplication) to the speed negotiated by the PHY. The MAC is unaware of
that and must remain configured for gigabit. If it is configured at
OCELOT_SPEED_10 or OCELOT_SPEED_100, it'll start transmitting PAUSE
frames out of control and never recover, _even if_ we then reconfigure
it at OCELOT_SPEED_1000 afterwards.

This patch fixes a bug that luckily did not have any functional impact.
We were writing 10, 100, 1000 etc into this 2-bit field in
DEV_CLOCK_CFG, but the hardware expects values in the range 0, 1, 2, 3.
So all speed values were getting truncated to 0, which is
OCELOT_SPEED_2500, and which also appears to be fine.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>