platform/x86: amd-pmc: Only report STB errors when STB enabled

platform/x86: amd-pmc: Only report STB errors when STB enabled

Currently if STB is disabled but an earlier function reported an
error an incorrect error will be emitted about failing to write to
STB.

Correct this logic error by only showing errors when STB is enabled.

Suggested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://lore.kernel.org/r/20220317190301.6818-1-mario.limonciello@amd.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>