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/nokia/mm-broadband-modem-nokia.c

@@ -266,7 +266,7 @@ retry_atz (GTask *task)
     ctx = g_task_get_task_data (task);
 
     mm_base_modem_at_command_full (self,
-                                   ctx->primary,
+                                   MM_IFACE_PORT_AT (ctx->primary),
                                    "Z",
                                    6,
                                    FALSE,