serial: rework 'connected' logic

So many modems just don't implement carrier detect that it makes
the previous 'connected' logic useless, so base it off connect/disconnect
and fix up a few places that didn't check connect status before
trying to send commands.  Also ensure the serial port is unlocked
for PPP to use when connected.

plugins/mm-modem-mbm.c

@@ -455,7 +455,7 @@ enable (MMModem *modem,
     } else {
         mm_serial_port_queue_command (primary, "+CREG=0", 100, NULL, NULL);
         mm_generic_gsm_pending_registration_stop (MM_GENERIC_GSM (modem));
-        if (mm_serial_port_is_connected (primary))
+        if (mm_port_get_connected (MM_PORT (primary)))
             mm_serial_port_flash (primary, 1000, disable_flash_done, info);
         else
             disable_flash_done (primary, info);

plugins/mm-modem-novatel-gsm.c

@@ -133,7 +133,7 @@ enable (MMModem *modem,
     g_assert (primary);
 
     if (!do_enable) {
-        if (mm_serial_port_is_connected (primary))
+        if (mm_port_get_connected (MM_PORT (primary)))
             mm_serial_port_flash (primary, 1000, disable_flash_done, info);
         else
             disable_flash_done (primary, info);

plugins/mm-modem-sierra.c

@@ -115,7 +115,7 @@ enable (MMModem *modem,
     g_assert (primary);
 
     if (!do_enable) {
-        if (mm_serial_port_is_connected (primary))
+        if (mm_port_get_connected (MM_PORT (primary)))
             mm_serial_port_flash (primary, 1000, disable_flash_done, info);
         else
             disable_flash_done (primary, info);

plugins/mm-modem-zte.c

@@ -134,7 +134,7 @@ enable (MMModem *modem,
     g_assert (primary);
 
     if (!do_enable) {
-        if (mm_serial_port_is_connected (primary))
+        if (mm_port_get_connected (MM_PORT (primary)))
             mm_serial_port_flash (primary, 1000, disable_flash_done, info);
         else
             disable_flash_done (primary, info);