igc: extract GRXRINGS from .get_rxnfc

igc: 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().

This simplifies the RX ring count retrieval and aligns igc with the new
ethtool API for querying RX ring parameters.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Link: https://patch.msgid.link/20251125-gxring_intel-v2-6-f55cd022d28b@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>