net: mvpp2: extract GRXRINGS from .get_rxnfc

net: mvpp2: extract GRXRINGS from .get_rxnfc

Commit 84eaf4359c36 ("net: ethtool: add get_rx_ring_count callback to
optimize RX ring queries") added specific support for GRXRINGS callback,
simplifying .get_rxnfc.

Remove the handling of GRXRINGS in .get_rxnfc() by moving it to the new
.get_rx_ring_count() for the mvpp2 driver.

This simplifies the RX ring count retrieval and aligns mvpp2 with the new
ethtool API for querying RX ring parameters, while keeping the other
rxnfc handlers (GRXCLSRLCNT, GRXCLSRULE, GRXCLSRLALL) intact.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20251121-marvell-v1-2-8338f3e55a4c@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>