usb: gadget: f_midi: enable use of the index parameter

The soundcard index to use for the ALSA device creation is passed as a
parameter to f_midi_bind_config(), but is assigned to midi->index only
after the call to f_midi_register_card(midi). So no matter what is passed
to f_midi_bind_config(), the actual index for snd_card_new() is always 0.
This probably works ok if at the moment of f_midi's bind there are no
other snd_cards, but if there are, it is not possible to bind f_midi.

This patch moves the assignment to a place before the call to
f_midi_register_card(midi).

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>