rapidio/tsi721: use PCI define for Max_Read_Request_Size

Replace a magic number with a PCI #define symbol.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Alexandre Bounine <alexandre.bounine@idt.com>