USB: gadget: midi: Fix deadlock unbinding driver while it is in use

Using snd_card_free_when_closed rather than snd_card_free in f_midi_unbind
makes it safe to disable the driver while a userspace client has the
ALSA device open.

Change-Id: Ibc40c01e7b1ce90fc61d3ea654b4816fadfc7ffd
Signed-off-by: Mike Lockwood <lockwood@google.com>