drm/nouveau/secboot/gm20b: fix the error return code in gm20b_secboot_tegra_read_wpr()

The error return code PTR_ERR(mc) is always 0 since mc is
equal to 0 in this error handling case.
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/secboot/gm20b.c

@@ -48,7 +48,7 @@ gm20b_secboot_tegra_read_wpr(struct gm200_secboot *gsb, u32 mc_base)
 	mc = ioremap(mc_base, 0xd00);
 	if (!mc) {
 		nvkm_error(&sb->subdev, "Cannot map Tegra MC registers\n");
-		return PTR_ERR(mc);
+		return -ENOMEM;
 	}
 	sb->wpr_addr = ioread32_native(mc + MC_SECURITY_CARVEOUT2_BOM_0) |
 	      ((u64)ioread32_native(mc + MC_SECURITY_CARVEOUT2_BOM_HI_0) << 32);