usb: gadget: f_midi: stash substream in gmidi_in_port structure

For every in_substream, there must be a corresponding gmidi_in_port
structure so it is perfectly viable and some might argue sensible to
stash pointer to the input substream in the gmidi_in_port structure.

This has an added benefit that if in_ports < MAX_PORTS, the whole
f_midi structure takes up less space because only in_ports number of
pointers for in_substream are allocated instead of MAX_PORTS lots of
them.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>