drm/amdgpu/swsmu/i2c: return an error if the SMU is not running

Return an error if someone tries to use the i2c bus when the
SMU is not running.  Otherwise we can end up sending commands
to the SMU which will either get ignored or could cause other
issues depending on what state the GPU and SMU are in.

Cc: Luben.Tuikov@amd.com
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>