seq: Clear UMP event flag for legacy apps

seq: Clear UMP event flag for legacy apps

It seems that some applications (at least Chrome WebMIDI) set random
bits to the flags of event packet, and this confuses as if they were
UMP-events, which are eventually filtered out.

Although it's a bug of applications, it's better to avoid the
regressions.  So this patch forcibly clears the UMP flag of the
incoming and outgoing events when the application is running in the
legacy mode (i.e. midi_version = 0).

Fixes: 2aefb5c41cc0 ("seq: Add UMP support")
Closes: https://github.com/alsa-project/alsa-lib/issues/360
Signed-off-by: Takashi Iwai <tiwai@suse.de>