net: ixgbe: convert to use .get_rx_ring_count

net: ixgbe: convert to use .get_rx_ring_count

Convert the ixgbe driver to use the new .get_rx_ring_count ethtool
operation for handling ETHTOOL_GRXRINGS command. This simplifies the
code by extracting the ring count logic into a dedicated callback.

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

This was compile-tested only.

Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20251113-ixgbe_gxrings-v2-1-0ecf57808a78@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>