ASoC: hdac_hdmi: Fix potential NULL dereference

Static checker warns:
Pointer 'hlink' returned from call to function 'snd_hdac_ext_bus_get_link'
at line may be NULL and will be dereferenced"

So we should always check the return of snd_hdac_ext_bus_get_link() before
referencing the link pointer

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>