ASoC: Intel: Skylake: Fix substream dereference before check

Smatch warns that we dereferenced substream before check, so fix
this by initializing ebus after the check

sound/soc/intel/skylake/skl-pcm.c:802 skl_get_position()
        warn: variable dereferenced before check 'substream->runtime'

Reported by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>