usb: gadget: f_midi: pre-allocate IN requests

This patch introduces pre-allocation of IN endpoint USB requests. This
improves on latency (requires no usb request allocation on transmit) and avoid
several potential probles on allocating too many usb requests (which involves
DMA pool allocation problems).

This implementation also handles better multiple MIDI Gadget ports, always
processing the last processed MIDI substream if the last USB request wasn't
enought to handle the whole stream.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>