mlx5: extract GRXRINGS from .get_rxnfc

mlx5: extract GRXRINGS from .get_rxnfc

Commit 84eaf4359c36 ("net: ethtool: add get_rx_ring_count callback to
optimize RX ring queries") added specific support for GRXRINGS callback,
simplifying .get_rxnfc.

Remove the handling of GRXRINGS in .get_rxnfc() by moving it to the new
.get_rx_ring_count() for both the mlx5 ethernet and IPoIB drivers.

The ETHTOOL_GRXRINGS handling was previously kept in .get_rxnfc() to
support "ethtool -x" when CONFIG_MLX5_EN_RXNFC=n. With the new
dedicated .get_rx_ring_count() callback, this is no longer necessary.

This simplifies the RX ring count retrieval and aligns mlx5 with the new
ethtool API for querying RX ring parameters.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20251113-mlx_grxrings-v1-2-0017f2af7dd0@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>