net: stmmac: move first xmit descriptor SARC and TBS config

net: stmmac: move first xmit descriptor SARC and TBS config

Move the first transmit descriptor's SARC and TBS configuration
alongside the code which populates the first descriptor, which helps
to keep all the code specific to that descriptor together.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w3d0H-0000000DfLp-0gIx@rmk-PC.armlinux.org.uk
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>