ASoC: rsnd: check return value of init function

Currently, this function cannot fail for the ADG case. Still, let's
apply defensive programming techniques to make sure we fail gracefully
whenever rsnd_mod_init() gets extended with another failure case.
Reported by Coverity (CID 1397893).

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Mark Brown <broonie@kernel.org>