bnxt_en: Fix 2 stray ethtool -S counters

The recent firmware interface change has added 2 counters in struct
rx_port_stats_ext. This caused 2 stray ethtool counters to be
displayed.

Since new counters are added from time to time, fix it so that the
ethtool logic will only display up to the maximum known counters.
These 2 counters are not used by production firmware yet.

Fixes: 754fbf60

 ("bnxt_en: Update firmware interface to 1.10.2.171")
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20231026013231.53271-1-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>