greybus: audio: Use standard API to set connections' private data

Use standard API greybus_set_drvdata() while setting private
data pointers for mgmt & data connection.

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