tg3: extract GRXRINGS from .get_rxnfc

tg3: extract GRXRINGS from .get_rxnfc

Commit 84eaf4359c36 ("net: ethtool: add get_rx_ring_count callback to
optimize RX ring queries") added specific support for GRXRINGS callback,
simplifying .get_rxnfc.

Remove the handling of GRXRINGS in .get_rxnfc() by moving it to the new
.get_rx_ring_count().

Given that tg3_get_rxnfc() only handles ETHTOOL_GRXRINGS, then this
function becomes useless now, and it is removed.

This also fixes the behavior for devices without MSIX support.
Previously, the function would return -EOPNOTSUPP, but now it correctly
returns 1.

The functionality remains the same: return the current queue count
if the device is running, otherwise return the minimum of online
CPUs and TG3_RSS_MAX_NUM_QS.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20251105-grxrings_v1-v1-1-54c2caafa1fd@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>