greybus: audio: Remove redundant lock protection & is_connected field

Each module maintains connected status & a lock to protect it.
Using codec->lock we can safely serialize ASoC specific callbacks
(in response to mixer_ctl update or dai_ops) and gb module
disconnect. Thus is_connected field can be removed.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>