ALSA: mtpav - Fix race in probe

Ingo reported Oops at probing mtpav driver.  It's a race between
the irq handler and the rawmidi instance registration.

This patch fixes the order of registration to avoid the race.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>