ASoC: Use card field to indicate whether a component is bound

Use the card field of a component to indicate whether it is bound or not.
This makes a certain sense given that the field contains the card the
component is bound to and a component can only be bound to one card at a
time. And it also requires to unset the card field when the component is
unbound from the card.

This makes the probded flag redundant and it can be removed.

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