mctp: Don't let RTM_DELROUTE delete local routes

We need to test against the existing route type, not
the rtm_type in the netlink request.

Fixes: 83f0a0b7 ("mctp: Specify route types, require rtm_type in RTM_*ROUTE messages")
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>