net: ethernet: ti: icssg_prueth: Remove Rx enable in init sequence

During startup, when network traffic is present and the PRU
firmware receives packets while the initialization is in process,
Rx stalls.

Fix this by moving the Rx enable from driver to firmware.

Signed-off-by: Ravi Gunasekaran <r-gunasekaran@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>