Currently, both the icssg-prueth and icssg-prueth-sr1 drivers create
a dedicated 'emac->cmd_wq' workqueue.
In the icssg-prueth-sr1 driver, this workqueue is not utilized at all.
In the icssg-prueth driver, the workqueue is only used to execute the
actual processing of ndo_set_rx_mode. However, creating a dedicated
workqueue for such a simple use case is unnecessary. To simplify the
code, switch to using the system default workqueue instead.