net: stmmac: move stmmac_xmit() skb head handling

net: stmmac: move stmmac_xmit() skb head handling

The skb head buffer handling is delayed in stmmac_xmit() until after
the skb fragments have been populated into the descriptors. The reason
is this code used to set the OWN bit on the first descriptor, which
then allows the TX DMA to process the first and subsequent descriptors.
However, as of commit 579a25a854d4 ("net: stmmac: Initial support for
TBS") this is now separated, but the comments weren't updated.

Move the code populating the first descriptor along side the jumbo code
which also populates the first descriptor. This gives a consistent
location where we populate the descriptor(s) for the SKB head.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w3d0C-0000000DfLj-0BLb@rmk-PC.armlinux.org.uk
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>