mctp: Do inits as a subsys_initcall

In a future change, we'll want to provide a registration call for
mctp-specific devices. This requires us to have the networks established
before device driver inits, so run the core init as a subsys_initcall.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>