usb: gadget: midi: Fix ignored "index" and "id" module parameters

The MIDI USB gadget driver has "index" and "id" parameters which should
determine the alsa sound card index and id to use when registering the
card. Those parameters had no effect, as the relevant information is only
set on the midi structure after f_midi_register_card is called.

This patch moves the two initialisation statements before the function call.

Signed-off-by: Marcus Weseloh <marcus@weseloh.cc>