net: dsa: felix: set proper pause frame timers based on link speed

state->speed holds a value of 10, 100, 1000 or 2500, but
SYS_MAC_FC_CFG_FC_LINK_SPEED expects a value in the range 0, 1, 2 or 3.

So set the correct speed encoding into this register.

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>