[ALSA] seq_midi_event: prevent running status after system messages

Reset the event type after encoding a system message to prevent any
following data bytes from being interpreted as data for a running status
system message, which is not allowed in MIDI.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>