greybus: audio: use variable 'is_connected' to maintain module state

there is race condition between _disconnect() request &
stop_trigger() in case of abrupt module removal.
And sometimes this can lead to deadlock while acquiring
codec_info->lock.
To avoid such situation, atomic variable is used to maintain
codec connected state.
During dai operations (trigger, shutdown, etc.), 'is_connected'
variable is validated to avoid unnecessary lock acquire in
case module already removed.

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