net: stmmac: Restrict warning on disabling DMA store and fwd mode

net: stmmac: Restrict warning on disabling DMA store and fwd mode

When setting 'snps,force_thresh_dma_mode' DT property, the following
warning is always emitted, regardless the status of force_sf_dma_mode:

dwmac-starfive 10020000.ethernet: force_sf_dma_mode is ignored if force_thresh_dma_mode is set.

Do not print the rather misleading message when DMA store and forward
mode is already disabled.

Fixes: e2a240c7d3bc ("driver:net:stmmac: Disable DMA store and forward mode if platform data force_thresh_dma_mode is set.")
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20230210202126.877548-1-cristian.ciocaltea@collabora.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>