fpga: dfl-pci: Use pci_find_vsec_capability() to simplify the code

PCI core add pci_find_vsec_capability() to query VSEC. We can use that
core API to simplify the code.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20230802080726.178194-1-wangxiongfeng2@huawei.com
Signed-off-by: Xu Yilun <yilun.xu@intel.com>