base-modem-at: switch to use the MMIfacePortAt by default

The whole codebase is updated to assume the MMBaseModem AT operations
require an object implementing the MMIfacePortAt interface, instead of
a MMPortSerialAt object.

In the places where we know the port is a MMPortSerialAt, e.g. when we
query explicitly for the primary or secondary serial AT port, we will
use an explicit MM_IFACE_PORT_AT() cast when calling the AT command
method.

In the places where we get_best_at_port() we're going to assume that
we don't know the port type. This method will be updated in a follow
up commit to support the MMIfacePortAt interface as well.

src/plugins/simtech/mm-shared-simtech.c

@@ -585,7 +585,7 @@ run_voice_enable_disable_unsolicited_events (GTask *task)
 
     if (port) {
         mm_base_modem_at_command_full (MM_BASE_MODEM (self),
-                                       port,
+                                       MM_IFACE_PORT_AT (port),
                                        ctx->clcc_command,
                                        3,
                                        FALSE,