bnxt_en: Add pcie_stat_len to struct bp

bnxt_en: Add pcie_stat_len to struct bp

Add this length field to capture the length of the pcie stats structure
supported by the FW.  This length will be determined in
bnxt_ethtool_init().  The minimum of this FW length and the length
known to the driver will determine the actual ethtool -d length.

Suggested-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Shruti Parab <shruti.parab@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20250819163919.104075-4-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>