net: ionic: convert to use .get_rx_ring_count

net: ionic: convert to use .get_rx_ring_count

Use the newly introduced .get_rx_ring_count ethtool ops callback instead
of handling ETHTOOL_GRXRINGS directly in .get_rxnfc().

Since ETHTOOL_GRXRINGS was the only command handled by ionic_get_rxnfc(),
remove the function entirely.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Link: https://patch.msgid.link/20260122-grxring_big_v4-v2-6-94dbe4dcaa10@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>