drm/amdgpu: ensure 0 is returned for success in jpeg_v2_5_wait_for_idle

In the cases where adev->jpeg.num_jpeg_inst is zero or the condition
adev->jpeg.harvest_config & (1 << i) is always non-zero the variable
ret is never set to an error condition and the function returns
an uninitialized value in ret.  Since the only exit condition at
the end if the function is a success then explicitly return
0 rather than a potentially uninitialized value in ret.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 14f43e8f ("drm/amdgpu: move JPEG2.5 out from VCN2.5")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>