net: ethtool: update set_rxfh to use ethtool_get_rx_ring_count helper

net: ethtool: update set_rxfh to use ethtool_get_rx_ring_count helper

Modify ethtool_set_rxfh() to use the new ethtool_get_rx_ring_count()
helper function for retrieving the number of RX rings instead of
directly calling get_rxnfc with ETHTOOL_GRXRINGS.

This way, we can leverage the new helper if it is available in ethtool_ops.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250917-gxrings-v4-5-dae520e2e1cb@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>