lpsink: Ignore async message from audio sink
lpsink: Ignore async message from audio sink

Audio sink element run as async-disable status.
However after doing seek, it emit async done message by itself.
Thus this patch will prevent it by drop async message at lpsink.

Change-Id: I6b2fb5332f7da39110b5bfc87d39874452bdd059
Reviewed-on: https://gpro.palm.com/27639
Tested-by: Wonchul Lee <9841bdbec9dbb2acc2dcaaa1dab0491399f0e464@lge.com>
Reviewed-by: HoonHee Lee <41497318ef67053f37c2ac0e974a3369f66d66ec@lge.com>
Reviewed-by: Jeongseok Kim <0c03ab7a1b32f29697716389c3f9549845834271@lge.com>
Tested-by: Jeongseok Kim <0c03ab7a1b32f29697716389c3f9549845834271@lge.com>

justinjoy/gst-plugins-lp,justinjoy/gst-plugins-lp