ASoC: samsung: spdif: Fix DMA filter initialization

This patch fixes issues introduced in commit 73f5dfc6


"ASoC: samsung: get access to DMA engine early to defer probe properly"
and indicated by a following compilation warning:

  CC [M]  sound/soc/samsung/spdif.o
sound/soc/samsung/spdif.c: In function ‘spdif_probe’:
sound/soc/samsung/spdif.c:419:6: warning: ‘filter’ may be used uninitialized
in this function [-Wuninitialized]

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>