ALSA: seq: Fix missing channel at encoding RPN/NRPN MIDI2 messages

The conversion from the legacy event to MIDI2 UMP for RPN and NRPN
missed the setup of the channel number, resulting in always the
channel 0.  Fix it.

Fixes: e9e02819

 ("ALSA: seq: Automatic conversion of UMP events")
Link: https://patch.msgid.link/20240625095200.25745-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>