drm/radeon/dpm: fix typo in EVERGREEN_SMC_FIRMWARE_HEADER_softRegisters

Should be at 0x8 rather than 0.

fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=60523

Noticed by ArtForz on #radeon

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org