bnxt_en: Rename switch_id to dsn

Instead of switch_id, renaming it to dsn will be more meaningful
so that it can be used to display device serial number in follow up
patch via devlink_info command.

Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>