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: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>