subsys/mgmt/mcumgr: Reduce unnecessary ROM usage

mcumgr's SMP UDP transport was unnecessarily using a potentially large
amount of ROM space due to static initialising fields in a
config struct that also contains buffers/stacks.

This has been changed to instead initialise fields in the start
function, reducing ROM usage by ~5K in the default configuration
with IPv4 and IPv6 enabled.

Signed-off-by: Ben Marsh <ben.marsh@helvar.com>