drm/amd/powerplay: ack the SMUToHost interrupt on receive V2

There will be no further interrupt without proper ack
for current one.

V2: fix typo to really set ACK bit only

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>