tg3: Fix num of RX queues being reported by ethtool

tg3: Fix num of RX queues being reported by ethtool

Using num_online_cpus() to report number of queues is actually not
correct, as reported by Michael[1].

netif_get_num_default_rss_queues() was used to replace num_online_cpus()
in the past, but tg3 ethtool callbacks didn't get converted. Doing it
now.

Link: https://lore.kernel.org/all/CACKFLim7ruspmqvjr6bNRq5Z_XXVk3vVaLZOons7kMCzsEG23A@mail.gmail.com/#t
Signed-off-by: Breno Leitao <leitao@debian.org>
Suggested-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20251107-tg3_counts-v1-1-337fe5c8ccb7@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>