net: ethtool: use the new helper in rss_set_prep_indir()

net: ethtool: use the new helper in rss_set_prep_indir()

Refactor rss_set_prep_indir() to utilize the new
ethtool_get_rx_ring_count() helper for determining the number of RX
rings, replacing the direct use of get_rxnfc with ETHTOOL_GRXRINGS.

This ensures compatibility with both legacy and new ethtool_ops
interfaces by transparently multiplexing between them.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250917-gxrings-v4-7-dae520e2e1cb@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>