ASoC: dapm: Do not add un-muxed paths to MUX control

Paths that are directly connected to a MUX widget are not affected by
changes to the MUX's control. Rather than checking if a path is directly
connected each time the MUX is updated do it only once when MUX is created.

We can also remove the check for e->texts[mux] != NULL, since if that
condition was true the code would have had already crashed much earlier (And
generally speaking if a enum's 'texts' entry is NULL it's a bug in the
driver).

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