staging: et131x: Use pci_dev->pcie_mpss and pcie_set_readrq() to simplify code

The PCI core caches the "PCIe Max Payload Size Supported" in
pci_dev->pcie_mpss, so use that instead of pcie_capability_read_dword().
Also use pcie_set_readrq() instead of pcie_capability_clear_and_set_word()
to simplify code.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Mark Einon <mark.einon@gmail.com>