ASoC: bcm: Fix pointer cast warning

The NULL check can be done gracefully without cast.  It fixes a
compile warning like:
  sound/soc/bcm/bcm63xx-pcm-whistler.c:184:6: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Fixes: 88eb404c ("ASoC: brcm: Add DSL/PON SoC audio driver")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200330135645.9707-1-tiwai@suse.de


Signed-off-by: Mark Brown <broonie@kernel.org>