EtherPauseFrame arrival during IFG is neglected

[rhornig]

Pause frame arrival is processed by EtherMACFullDuplex::processPauseCommand() function. When the transmitter is in the WAIT_IFG_STATE the pauseUnitsRequested variable is updated. However, this variable doesn't get checked in the EtherMACFullDuplex::handleEndIFGPeriod() function and the next frame is being transmitted.