drivers: ethernet: Modify RX thread creation and update KConfig

This change will allow users to configure the Ethernet RX thread
according to their specific real-time requirements.
Adding preemptive threading helps to reduce jitter and
the impact of Ethernet traffic on real-time performance.

Signed-off-by: IBEN EL HADJ MESSAOUD Marwa <marwa.ibenelhadjmessaoud-ext@st.com>