ASoC: simple-card: fix possible uninitialized single_cpu local variable

The 'single_cpu' local variable is assigned by asoc_simple_parse_dai()
and later used in a asoc_simple_canonicalize_cpu() call, assuming the
entire function did not exit on errors.

However the first function returns 0 if passed device_node is NULL,
thus leaving the variable uninitialized and reporting success.

Addresses-Coverity: Uninitialized scalar variable
Fixes: 8f7f298a

 ("ASoC: simple-card-utils: separate asoc_simple_card_parse_dai()")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Sameer Pujar <spujar@nvidia.com>
Link: https://lore.kernel.org/r/20210407092027.60769-1-krzysztof.kozlowski@canonical.com
Signed-off-by: Mark Brown <broonie@kernel.org>