mgmt: mcumgr: transport: smp: Fix smp_transport_clients list init.

Make sure smp_transport_clients list is only initialized once and
before any transports will try to register and add entries to this
list.

The smp_init() routine was called after smp_init_uart(), causing
the list to be emptied again after registration of the uart client
transport.

Signed-off-by: Bas van Loon <bas@arch-embedded.com>