net: designware: fix tx packet length

net: designware: fix tx packet length

The designware driver has a bug in setting the tx length into the dma
descriptor: it always or's the length into the descriptor without
zeroing out the length mask before.

This results in occasional packets being transmitted with a length
greater than they should be (trailer). Due to the nature of Ethernet
allowing such a trailer, most packets seem to be parsed fine by remote
hosts, which is probably why this hasn't been noticed.

Fix this by correctly clearing the size mask before setting the new
length.

Tested on socfpga gen5.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>