a52: Ignore start request if we're already running

When trying to start a PCM that's already running, the result is EBADFD.
We might have implicitly started the buffer by filling it up,
so just ignore this request if we're already running.

Reported-by: Chow Loong Jin <hyperair@ubuntu.com>
Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>