audiod: fix receiver restart delay

audiod: fix receiver restart delay

The current code started a new receiver node for the current format
only if the receiver_node is NULL for each slot which streams
that format. Since that pointer is only set to NULL _after_ the
writer/filters are closed, audiod did not start the new receiver
early enough.

Fix is simple: Check also the eof field in struct receiver_node.
As we're at it, rename decoder_running() to receiver_running() and
move it straight above its single caller.

As a consequence of this change, the restart_barrier has to be set
already in rn_event_handler() as this is the place where rn->eof is
set to one.