USB: gadget: f_midi: allow a dynamic number of input and output ports

The code in the MIDI gadget was already sort of prepared for multi-port
configuration, so the streaming logic itself didn't need much tweaking.
However, the descriptors change when the number of ports do, and so some
rework of the the preparation algorithms were necessary.

Successfully tested on Linux and Max OS X hosts for both input and
output streams.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>