ASoC: topology: Fix endianness issue

As done in already existing cases, we should use le32_to_cpu macro while
accessing hdr->magic. Found with sparse.

Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Link: https://lore.kernel.org/r/20200415162435.31859-2-amadeuszx.slawinski@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>