drm/nouveau/secboot/gm20b: initialize pointer in gm20b_secboot_new()

We accidentally set "psb" which is a no-op instead of "*psb" so it
generates a static checker warning.  We should probably set it before
the first error return so that it's always initialized.

Fixes: 923f1bd2 ("drm/nouveau/secboot/gm20b: add secure boot support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>