net: dsa: felix: convert TAS link speed based on phylink speed

state->speed holds a value of 10, 100, 1000 or 2500, but
QSYS_TAG_CONFIG_LINK_SPEED expects a value of 0, 1, 2, 3. So convert the
speed to a proper value.

Fixes: de143c0e ("net: dsa: felix: Configure Time-Aware Scheduler via taprio offload")
Signed-off-by: Xiaoliang Yang <xiaoliang.yang_1@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>