drm/amdgpu: Clean up errors in mmhub_v9_4.c

Fix the following errors reported by checkpatch:

ERROR: code indent should use tabs where possible
ERROR: space required before the open parenthesis '('

Signed-off-by: Ran Sun <sunran001@208suo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>