bnxt_en: Read VPD info only for PFs

Virtual functions does not have VPD information. This patch modifies
calling bnxt_read_vpd_info() only for PFs and avoids an unnecessary
error log.

Fixes: a0d0fd70 ("bnxt_en: Read partno and serialno of the board from VPD")
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>