bnxt_en: Device serial number is supported only for PFs.

Don't read DSN on VFs that do not have the PCI capability.

Fixes: 03213a99 ("bnxt: move bp->switch_id initialization to PF probe")
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>