drm/nouveau/i2c: fix typo when checking nvio i2c port validity

Reported-by: Mathieu Chouquet-Stringer <mathieu@csetco.com>
Tested-by: Mathieu Chouquet-Stringer <mathieu@csetco.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>