ALSA: seq: ump: Optimize conversions from SysEx to UMP

The current conversion from the legacy SysEx event to UMP SysEx packet
in the sequencer core has a couple of issues:

* The first packet trims the SysEx start byte (0xf0), hence it
  contains only 5 bytes instead of 6.  This isn't wrong, per
  specification, but it's strange not to fill 6 bytes.

* When the SysEx end marker (0xf7) is placed at the first byte of the
  next packet, it'll end up with an empty data just with the END
  status.  It can be rather folded into the previous packet with the
  END status.

This patch tries to address those issues.  The first packet may have 6
bytes even with the SysEx start, and an empty packet with the SysEx
end marker is omitted.

Fixes: e9e02819

 ("ALSA: seq: Automatic conversion of UMP events")
Cc: <stable@vger.kernel.org>
Link: https://patch.msgid.link/20240726143455.3254-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>