net: stmmac: move stmmac_xmit() initial variable init

Move tx_q, first_tx and txq_stats just before their first use.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1w3d0R-0000000DfM1-1itN@rmk-PC.armlinux.org.uk
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>