Reduce jitter of transmitted IO frames

During the low cost performance (which is conducted on the EtherNet/IP PlugFest) the jitter of the transmitted IO frames of a device is measured. Currently the stack passes this test, so there is actually no urgent need for improvement. However, when the stack is tested with an RPI value of 1ms the jitter comes close to what is acceptable. Therefore, the handling of transmitted IO frames shall be improved.