ethtool: remove ETHTOOL_GRXRINGS fallback through get_rxnfc

ethtool: remove ETHTOOL_GRXRINGS fallback through get_rxnfc

All drivers that need to report the RX ring count now implement the
get_rx_ring_count callback directly. Remove the legacy fallback path
that obtained this information by calling get_rxnfc with ETHTOOL_GRXRINGS.

This simplifies the code and makes get_rx_ring_count the only way
to retrieve the RX ring count.

Note: ethtool_get_rx_ring_count() returns int to allow returning
-EOPNOTSUPP, while the callback returns u32. The implicit conversion
is safe since RX ring counts will not exceed INT_MAX while we are still
alive.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260126-grxring_final-v1-1-0981cb24512e@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>