net: enetc: convert to use .get_rx_ring_count

net: enetc: convert to use .get_rx_ring_count

Convert the enetc 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 in two ways:

1. For enetc_get_rxnfc(): Remove the ETHTOOL_GRXRINGS case from the
   switch statement while keeping other cases for classifier rules.

2. For enetc4_get_rxnfc(): Remove it completely and use
   enetc_get_rxnfc() instead.

Now on, enetc_get_rx_ring_count() is the callback that returns the
number of RX rings for enetc driver.

Also, remove the documentation around enetc4_get_rxnfc(), which was not
matching what the function did(?!).

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251128-gxring_freescale-v1-3-22a978abf29e@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>