dsa: Allow core and drivers to be built as modules

Change the kconfig types to tristate and adjust the condition for
declaring net_device::dsa_ptr to allow for this.

Adjust the makefile so that if NET_DSA_MV88E6123_61_65=y and
NET_DSA_MV88E6131=m or vice versa then both drivers are built-in.  We
could leave these options as bool and make NET_DSA_MV88E6XXX a
user-selected option, but that would break existing configurations.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>