PCI/PME: Replace RMW of Root Status register with direct write

PCI/PME: Replace RMW of Root Status register with direct write

As of PCIe r7.0, the Root Status register contains a single writeable bit
(PME Status, type RW1C) and otherwise just read-only bits and RsvdZ bits
(which software must write as zero, PCIe r7.0 sec 7.4).

Thus, when clearing the PME Status bit, there's no need to perform a
read-modify-write of the register.  Instead, the bit can be written
directly.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://patch.msgid.link/39f87c99f6c44be3c0371c79e454e6fde7be0d4d.1761497583.git.lukas@wunner.de