rapidio/tsi721: use PCI_EXP_DEVCTL2_COMP_TIMEOUT macro

Use the existing PCI_EXP_DEVCTL2_COMP_TIMEOUT macro instead of hard-coding
the PCIe Completion Timeout Value mask.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Alexandre Bounine <alexandre.bounine@idt.com>