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.c

@@ -69,6 +69,8 @@ mm_modem_error_get_type (void)
             ENUM_ENTRY (MM_MODEM_ERROR_GENERAL,                 "General"),
             ENUM_ENTRY (MM_MODEM_ERROR_OPERATION_NOT_SUPPORTED, "OperationNotSupported"),
             ENUM_ENTRY (MM_MODEM_ERROR_CONNECTED,               "Connected"),
+            ENUM_ENTRY (MM_MODEM_ERROR_DISCONNECTED,            "Disconnected"),
+            ENUM_ENTRY (MM_MODEM_ERROR_OPERATION_IN_PROGRESS,   "OperationInProgress"),
             { 0, 0, 0 }
         };