usb: gadget: f_midi: move some of f_midi_transmit to separate func

Move some of the f_midi_transmit to a separate f_midi_do_transmit
function so the massive indention levels are not so jarring.  This
introduces no changes in behaviour.

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