core: don't allow concurrent flashes on the same device

Previously, a few operations (like disable) could trigger a modem
flash in parallel with another flash.  That's wrong, don't allow
that.  At the same time, add in finer-grained error checking on
serial port speed operations, and fix a GSM generic bug that would
send the POWER_UP string on disable.

src/mm-errors.h

@@ -35,7 +35,9 @@ GType  mm_serial_error_get_type (void);
 enum {
     MM_MODEM_ERROR_GENERAL = 0,
     MM_MODEM_ERROR_OPERATION_NOT_SUPPORTED = 1,
-    MM_MODEM_ERROR_CONNECTED = 2
+    MM_MODEM_ERROR_CONNECTED = 2,
+    MM_MODEM_ERROR_DISCONNECTED = 3,
+    MM_MODEM_ERROR_OPERATION_IN_PROGRESS = 4
 };
 
 #define MM_MODEM_ERROR (mm_modem_error_quark ())