Set PCIe device's Max_Payload_Size to match PCIe root's.

Usually on boot the MPS is already configured by BIOS. But we've
found that on hot-plug it is not true at least for our Supermicro
X11 boards. As result, mismatch between root's configuration of
256 bytes and device's default of 128 bytes cause problems for some
devices, while others seem to work fine.

MFC after: 1 month
Sponsored by: iXsystems, Inc.