ASoC: dapm: Fix NULL pointer dereference when registering card with widgets

Commit 0bd2ac3d ("ASoC: Remove CODEC pointer from snd_soc_dapm_context")
introduced regression to snd_soc_dapm_new_controls() when registering a card
with card->dapm_widgets set. Call chain is:

    snd_soc_register_card()
    -> snd_soc_instantiate_card()
       -> snd_soc_dapm_new_controls()
          -> snd_soc_dapm_new_control()

Null pointer dereference occurs since card->dapm context doesn't have
associated component. Fix this by setting widget codec pointer
conditionally.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>