ALSA: ump: Explicitly reset RPN with Null RPN

RPN with 127:127 is treated as a Null RPN, just to reset the
parameters, and it's not translated to MIDI2.  Although the current
code can work as is in most cases, better to implement the RPN reset
explicitly for Null message.

Link: https://patch.msgid.link/20240731130528.12600-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>