fix(modem): Fix DUAL_MODE regression from cb6e03a

In the recent DTE refactoring (cb6e03a) we install terminal callbacks
in the constructor, but the change missed initializing modem_state
in DTE constructors which take two terminals as arguments to work in
DUAL mode.

Author: david-cermak

components/esp_modem/src/esp_modem_dte.cpp

@@ -33,15 +33,15 @@ DTE::DTE(std::unique_ptr<Terminal> terminal):
 DTE::DTE(const esp_modem_dte_config *config, std::unique_ptr<Terminal> t, std::unique_ptr<Terminal> s):
     buffer(config->dte_buffer_size),
     cmux_term(nullptr), primary_term(std::move(t)), secondary_term(std::move(s)),
-    mode(modem_mode::UNDEF)
+    mode(modem_mode::DUAL_MODE)
 {
     set_command_callbacks();
 }
 
 DTE::DTE(std::unique_ptr<Terminal> t, std::unique_ptr<Terminal> s):
     buffer(dte_default_buffer_size),
     cmux_term(nullptr), primary_term(std::move(t)), secondary_term(std::move(s)),
-    mode(modem_mode::UNDEF)
+    mode(modem_mode::DUAL_MODE)
 {
     set_command_callbacks();
 }