net: dsa: b53: Fix build with B53_SRAB enabled and not B53_SERDES

In case B53_SRAB is enabled, but not B53_SERDES, we can get the
following linking error:

ERROR: "b53_serdes_init" [drivers/net/dsa/b53/b53_srab.ko] undefined!

We also need to ifdef the body of b53_srab_serdes_map_lane() since it
would not be used when B53_SERDES is disabled and that would produce a
warning.

Fixes: 0e01491d

 ("net: dsa: b53: Add SerDes support")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>