cxgb4: Simplify PCIe Completion Timeout setting

Simplify PCIe Completion Timeout setting by using the
pcie_capability_clear_and_set_word() interface.  No functional change
intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>