drm/amdgpu: fix missing assignment of error return code to variable ret

An earlier commit replaced ttm_bo_wait with amdgpu_bo_sync_wait and
removed the error return assignment to variable ret. Fix this by adding
the assignment back. Also break line to clean up checkpatch overly
long line warning.

Detected by CoverityScan, CID#1477327 ("Logically dead code")

Fixes: c60cd590 ("drm/amdgpu: Replace ttm_bo_wait with amdgpu_bo_sync_wait")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>