fix A/52 spdif output regression

fix A/52 spdif output regression

A/52 spdif output was broken by the switch over to the new filter API.

  * a52tofloat32 should only take control if there is _no_ spdif
    output requested otherwise the decoded stream ends up on the spdif
    device

  * a52tospdif has to set a pts/dts value otherwise the frames get
    (wrongfully) discarded as too late because a wrong pts/dts value
    got evaluated by aout_OutputNextBuffer()

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>