PCI/DPC: Use defined fields with DPC_CTL register

Instead of using a literal to clear bits, add PCI_EXP_DPC_CTL_EN_MASK
and use the usual pattern to modify a bitfield.

While at it, rearrange RMW code more logically together.

Link: https://lore.kernel.org/r/20231018113254.17616-6-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>