ASoC: core: remove unused variable in soc_probe() in linux-next

With commit 28d528c8 "ASoC: core: Remove pointless error on card
registration failure", the variable ret is no longer used in
soc_probe() and generates an unused variable warning during a build.

Signed-off-by: Jerry Snitselaar <dev@snitselaar.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>