net: stmmac: selftests: Switch to dev_direct_xmit()

In the upcoming commit for TBS selftest we will need to send a packet on
a specific Queue. As stmmac fallsback to netdev_pick_tx() on the select
Queue callback, we need to switch all selftests logic to
dev_direct_xmit() so that we can send the given SKB on a specific Queue.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>