bnxt_en: Skip reading PXP registers during ethtool -d if unsupported

bnxt_en: Skip reading PXP registers during ethtool -d if unsupported

Newer firmware does not allow reading the PXP registers during
ethtool -d, so skip the firmware call in that case.  Userspace
(bnxt.c) always expects the register block to be populated so
zeroes will be returned instead.

Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20241217182620.2454075-6-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>