[ALSA] seq-midi - silently ignore non-MIDI events

ALSA sequencer
When non-MIDI sequencer events are sent to a RawMIDI port, silently
ignore them instead of returning a confusing error code which may upset
the sequencer and abort the current write() to /dev/snd/seq.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>