ASoC: dapm: Check for NULL widget in dapm_update_dai_unlocked

DAIs linked to the dummy will not have an associated playback/capture
widget, so we need to skip the update in that case.

Fixes: 078a85f2

 ("ASoC: dapm: Only power up active channels from a DAI")
Reported-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Tested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Tested-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>