drm/nouveau/pci: Move an expression into a function call parameter in nvkm_pcie_set_link()

The variable “pbus” was read only once in the implementation of
the function “nvkm_pcie_set_link”.
Thus move the usage of an expression into a parameter for a function call.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/04e2c1f5-26c7-9a23-5861-3d8757556cdd@web.de