Reported by smatch:
amdgpu_atombios_i2c_process_i2c_ch() error: we previously assumed 'buf' could be null

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>