net: mvneta: convert to use .get_rx_ring_count

net: mvneta: convert to use .get_rx_ring_count

Convert the mvneta 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/20251121-marvell-v1-1-8338f3e55a4c@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>