drm/nouveau/pci: Move a variable assignment behind condition checks in nvkm_pcie_set_...

drm/nouveau/pci: Move a variable assignment behind condition checks in nvkm_pcie_set_link()

The address of a data structure member was determined before
a corresponding null pointer check in the implementation of
the function “nvkm_pcie_set_link”.

Thus avoid the risk for undefined behaviour by moving the assignment
for the variable “subdev” behind some condition checks.

This issue was detected by using the Coccinelle software.

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/b31019b1-bb73-ed93-3378-e551e17a4f32@web.de