net: ethernet: stmmac: fix documentation warning

This patch remove the following documentation warning
drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:103: warning: Excess function parameter 'priv' description in 'stmmac_axi_setup'
It was introduced in commit afea0365

 ("stmmac: rework DMA bus setting and introduce new platform AXI structure")

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>