net: dsa: mv88e6xxx: simplify SERDES code for Topaz and Peridot

By adding an additional serdes_get_lane implementation (for Topaz), we
can merge the implementations of other SERDES functions (powering and
IRQs). We can skip checking port numbers, since the serdes_get_lane()
methods inform if there is no lane on a port or if the lane cannot be
used for given cmode.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>