ALSA: seq: Fix delivery of UMP events to group ports

ALSA: seq: Fix delivery of UMP events to group ports

When an event with UMP message is sent to a UMP client, the EP port
receives always no matter where the event is sent to, as it's a
catch-all port.  OTOH, if an event is sent to EP port, and if the
event has a certain UMP Group, it should have been delivered to the
associated UMP Group port, too, but this was ignored, so far.

This patch addresses the behavior.  Now a UMP event sent to the
Endpoint port will be delivered to the subscribers of the UMP group
port the event is associated with.

The patch also does a bit of refactoring to simplify the code about
__deliver_to_subscribers().

Fixes: 177ccf811df4 ("ALSA: seq: Support MIDI 2.0 UMP Endpoint port")
Link: https://patch.msgid.link/20250511134528.6314-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>