ASoC: dapm: Drop always true checks

list_first_entry() always returns non NULL and since the code previously
checked that list is not empty it will also be a valid pointer. Furthermore
a path has always a sink or a source widget. So both checks are redundant
and can be removed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>