net: stmmac: convert to use .get_rx_ring_count

net: stmmac: convert to use .get_rx_ring_count

Convert the stmmac driver to use the new .get_rx_ring_count
ethtool operation instead of implementing .get_rxnfc for handling
ETHTOOL_GRXRINGS command.

Since stmmac_get_rxnfc() only handled ETHTOOL_GRXRINGS (returning
-EOPNOTSUPP for all other commands), remove it entirely and replace
it with the simpler stmmac_get_rx_ring_count() callback.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260108-gxring_stmicro-v2-1-3dcadc8ed29b@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>