net: ena: convert to use .get_rx_ring_count

net: ena: 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 useful command handled by
ena_get_rxnfc(), remove the function entirely.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Arthur Kiyanovski <akiyano@amazon.com>
Link: https://patch.msgid.link/20260115-grxring_big_v2-v1-4-b3e1b58bced5@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>