net: mvneta: zero Tx descriptors on init

Make the initialization sequence consistent with the Linux kernel
driver.

Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>