ASoC: fix NULL pointer dereference in soc_suspend()

In case the initalization of an soc_device failed, there is no codec
associated with it. soc_suspend() will still dereference the pointer
and cause an Ooops when entering the sleep mode.

This happens on our board with a multi-target kernel image when booted
on a machine without audio circuits.

This patch makes the code bail out very early in this special case.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>