ALSA: usb-audio: Manage number of rawmidis globally

We're going to create rawmidi objects for MIDI 2.0 in a different code
from the current code for USB-MIDI 1.0.  As a preliminary work, this
patch adds the number of rawmidi objects to keep globally in a
USB-audio card instance, so that it can be referred from both MIDI 1.0
and 2.0 code.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-8-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>