net: dsa: mv88e6xxx: Fix 88E6141/6341 2500mbps SERDES speed

This is a fix for the port_set_speed method for the Topaz family.
Currently the same method is used as for the Peridot family, but
this is wrong for the SERDES port.

On Topaz, the SERDES port is port 5, not 9 and 10 as in Peridot.
Moreover setting alt_bit on Topaz only makes sense for port 0 (for
(differentiating 100mbps vs 200mbps). The SERDES port does not
support more than 2500mbps, so alt_bit does not make any difference.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>