net: designware: clear padding bytes

Short frames are padded to the minimum allowed size of 60 bytes.
However, the designware driver sends old data in these padding bytes.
It is common practice to zero out these padding bytes ro prevent
leaking memory contents to other hosts.

Fix the padding code to zero out the padded bytes at the end.

Tested on socfpga gen5.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>