PCI: Add defines for PCIe Max_Read_Request_Size

There are a few drivers using magic numbers when operating with PCIe
capabilities and PCI_EXP_DEVCTL_READRQ.  Define known values to allow
cleaning their code a bit.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>