net: gve: convert to use .get_rx_ring_count

net: gve: convert to use .get_rx_ring_count

Convert the Google Virtual Ethernet (GVE) driver to use the new
.get_rx_ring_count ethtool operation instead of handling
ETHTOOL_GRXRINGS in .get_rxnfc. This simplifies the code by moving the
ring count query to a dedicated callback.

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

Reviewed-by: Subbaraya Sundeep <sbhatta@marvell.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260105-gxring_google-v2-1-e7cfe924d429@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>