pcie: introduce pcie_sltctl_powered_off() helper

pcie: introduce pcie_sltctl_powered_off() helper

In pcie_cap_slot_write_config() we check for PCI_EXP_SLTCTL_PWR_OFF in
a bad form. We should distinguish PCI_EXP_SLTCTL_PWR which is a "mask"
and PCI_EXP_SLTCTL_PWR_OFF which is value for that mask.

Better code is in pcie_cap_slot_unplug_request_cb() and in
pcie_cap_update_power(). Let's use same pattern everywhere. To simplify
things add also a helper.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Kuchin <antonkuchin@yandex-team.ru>
Message-Id: <20230216180356.156832-12-vsementsov@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>