net: gianfar: convert to use .get_rx_ring_count

net: gianfar: convert to use .get_rx_ring_count

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

The driver still maintains .get_rxnfc for other commands including
ETHTOOL_GRXCLSRLCNT, ETHTOOL_GRXCLSRULE, and ETHTOOL_GRXCLSRLALL.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251128-gxring_freescale-v1-1-22a978abf29e@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>