net: thunder: convert to use .get_rx_ring_count

net: thunder: convert to use .get_rx_ring_count

Convert the Cavium Thunder NIC VF driver to use the new .get_rx_ring_count
ethtool operation instead of implementing .get_rxnfc solely for handling
ETHTOOL_GRXRINGS command. This simplifies the code by removing the
switch statement and replacing it with a direct return of the queue
count.

The new callback provides the same functionality in a more direct way,
following the ongoing ethtool API modernization.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20251126-gxring_cavium-v1-1-a066c0c9e0c6@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>