mlxsw: spectrum: Assume CONFIG_NET_DEVLINK is always enabled

Since commit f6b19b35

 ("net: devlink: select NET_DEVLINK
from drivers") adds implicit select of NET_DEVLINK for
mlxsw, the code does not have to deal with the case
when CONFIG_NET_DEVLINK is not enabled. So remove the ifcase
and adjust Makefile.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>