bnxt_en: Add board.serial_number field to info_get cb

Add board.serial_number field info to info_get cb via devlink,
if driver can fetch the information from the device.

Cc: Jiri Pirko <jiri@mellanox.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>