pcie: pcie_cap_slot_write_config(): use correct macro

pcie: pcie_cap_slot_write_config(): use correct macro

PCI_EXP_SLTCTL_PIC_OFF is a value, and PCI_EXP_SLTCTL_PIC is a mask.
Happily PCI_EXP_SLTCTL_PIC_OFF is a maximum value for this mask and is
equal to the mask itself. Still the code looks like a bug. Let's make
it more reader-friendly.

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: <20230216180356.156832-8-vsementsov@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>