net: benet: convert to use .get_rx_ring_count

net: benet: 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 be_get_rxnfc(),
remove the function entirely.

Since the be_multi_rxq() check in be_get_rxnfc() previously blocked RSS
configuration on single-queue setups (via ethtool core validation), add
an equivalent check to be_set_rxfh() to preserve this behavior, as
suggested by Jakub.

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-v2-1-94dbe4dcaa10@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>