net: mvneta: dcache flush TX descriptors at init

This fixes sporadic timeout on initial packet Tx (usually ARP), with an
error message like:

  timeout: packet not sent

Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Chris Packham <judge.packham@gmail.com>
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>